So, first things first, the majority of this overly long expository drop is going to be talking about the Moon Lord. The rest is about in-game balancing and what it feels like happened with that.
The Moon Lord himself, as a boss, is insane. This is totally fine--it's great, even! I love that the final boss of the game is huge, has multiple attack patterns, and when you defeat a segment, it doesn't stop him from using his attacks on you. It makes the fight challenging and thoroughly enjoyable to defeat him. There are, however, several major problems with this--and I feel like the latest patch to address them did it all wrong and backfired.
First, Spectre Armor with the Hood. Obviously this was the intent of the 1.3.03 update's debuff grant to Moony--its sole intent was to stop players easily farming him over and over with nothing but the Spectre Hood and a decent DPS magic weapon. Having killed him six times with ease using this setup and a Razorblade Typhoon, I can attest that it felt too easy, and that he needed some way to counteract this being the most reliable method of killing him. However...nullifying the Spectre Hood's ability entirely for the full duration of the fight, which is true only because he uses his tether-drain attack often enough the debuff never goes away if you're not somehow miraculously faster than him, is a kick in the teeth for the mage class. It wouldn't be that bad at all, except that his constant barrage of attacks, which come from unkillable sources once you've beaten the body part that originally used them, means taking damage is not only almost wholly unavoidable, it's also constant and forces you to fly around dodging for the full duration, lest your squishy mage die due to lower defense. Autohealing was a great solution to having such low defense, but none of the other classes really had it--the Spectre Hood heals players for approximately 12% of their weapon's base damage due to its damage cut and healing percentage. The Vampire Knives, the ONLY OTHER AUTOHEAL IN THE GAME, heal 7.5%. Their damage is low and forces you to use a single weapon with specific mechanics to heal--the Spectre Hood lets you use anything, from the Typhoon to the Razorpine to the Blizzard Staff to the Bubble Gun. Your DPS and thus your healing, whether your attacks home, whether they pierce, how much mana they use--it's all up to you and the healing results no matter what you go with. It opens a massive amount of player-build options and using different tools to solve different problems, rather than one slightly-clunky set of throwing knives and nothing else.
Giving the final boss, with 145,000 health and a constant barrage of high-damage attacks without any option to destroy projectiles or the sources of the attacks themselves, the ability to nullify the mage class's best set of gear was a direct brick to the face to anyone using it, seemingly because that option was the best and unbalanced because of the lack of other same-level autohealing options for other classes. The replacement new-endgame armor for the mage class instead boosts natural life regen by producing randomly-generated buffs that stack in three levels and last for around ten seconds a level if not replenished. This would be a great solution, except the increased life regen still doesn't hold a candle to the amount you can heal with a Hood and high-DPS weapon. The increased defense is nice, but not enough to offset the massive drop in survivability--meaning the Spectre Hood set is still the best mage outfit in the game for surviving heavy-damage environments, since you don't need to avoid as much damage as with the Nebula armor. The fact that the mage's new endgame armor is less effective than its previous endgame armor set is a clear sign of a poorly-balanced system. The weapons only make it worse. With my current build and Spectre Hood armor, my Typhoon deals 62 damage and costs 12 mana. The new Nebula Blaze weapon deals 112 damage and costs 13 mana. The Blaze fires slightly faster, but the Typhoon now fires TWO projectiles for one use. On top of that, while both are homing and move at around the same speed, the Blaze explodes on impact and the Typhoon can infinite-pierce and infinite-ricochet, lasting three seconds and dealing its 62 damage each time it hits during that duration--for each of the two wheels. There's no contest, the Typhoon--a previously endgame weapon--is vastly better than the new endgame weapon.
This only serves to highlight what feels like an egregious oversight by the development team: Autohealing is the current most viable solution for surviving endgame fights of all kinds, and the new endgame gear is less effective on all fronts for the one class that has a workable autoheal mechanic. The new Summoner gear is fantastic, since it gives much better defense and the set bonus is quite useful, as well as the two endgame minion weapons being particularly effective, but the minions attacking enemies around you still leaves you personally vulnerable and left to your own devices, most of which are base-damage weapons of whatever kind due to favoring Warding accessories and only using the ones that boost the minion cap. The new Ranged gear feels like a light rehash of the old endgame setup--a more reasonable stealth ability on the armor, which is all-around boosting instead of having multiple helmets; a bow that acts like a slightly-tweaked version of the Tsunami; a machine-gun that acts like a slightly better Chain Gun. The Melee gear is about the same thing: the armor is a different version of Beetle armor with only one type of buff, the new weapons are a thrown lasting-damage spear and a flail/chain-knife that looks like an extendable sword, and the yoyos are a great addition--but even the Terrarian pales in comparison to the Flairon with its homing projectiles and DPS abilities, given how far they travel and how well they home in. The mage armor grants a mana-regen buff, a health-regen buff and a damage booster buff for dealing damage, which stack and in a decent fight can be almost infinitely kept at their highest levels--which makes it a formidable player-damage-boosting and survivability-enhancing armor set. It feels like it's a level ahead of the other three armor sets, and that was the same truth in the last endgame sets with Spectre being the only one with autohealing. On top of that, the Spectre armor set is still a more viable alternative! Autohealing is clearly the way to go with things, and only the mage class has it in a workable fashion.
The easiest way to rebalance the classes so that mages weren't on top of everything with autohealing wouldn't have been to do what was done and add an alternate form of health gain as a result of damage for the class, since the other three classes still didn't get anything in that similar vein. It would have been to give the other three classes a similar ability on a similar level. The easiest way to balance autohealing would have been, and still would be, to create something like an accessory that heals players for, say, even 5% of the damage they deal. This would grant autohealing on a reduced level to summoners, rangers, and melee users, as well as keeping it with the mages--but it would allow for weapon versatility on a level that would give them all a reason to use the accessory in their builds, and the mages would drop Spectre Armor for the new accessory because the new armor would provide better defense and some different bonuses while the accessory kept their coveted autoheal. Mages would no longer be on top, but even with the rest, because autohealing would be available in a widespread fashion--and the reduced level of healing, countered by a lack of dependence on a particular weapon or armor set for it, would balance out against the Spectre Hood getup, making it no longer the best option. Endgame bosses and enemies from things like the Lunar Events could be given additional defense or the like, perhaps the Lunar Pillars and Moon Lord could get a debuff active during their full fights based on proximity that would reduce autohealing by a percentage instead of outright killing it, and gain additional damage to compensate for the player's ability to rapidly heal from it. Certain enemies could have the ability to inflict a debuff on attack that would nullify autohealing for, say, three to five seconds, providing a challenge in dodging as well as managing one's health and damage efficiently so as to not be caught off-guard by the debuff and left high and dry.
What was done instead was that for the final boss fight alone, for the final boss fight alone and none of the pillar fights or any previous areas of the game, autohealing was nullified completely. Every situation except the Moon Lord's fight is now best dealt with using the autohealing mage outfit, exactly as before, and the only thing that has changed is that the Moon Lord is now the exception to that rule. None of the other classes were affected--for them, the fight is exactly as hard as it used to be, which is the same difficulty that drove players to use the Spectre set as a better option--but for mages the fight is now cripplingly more difficult and requires the use of the new, less-efficient armor set to realistically stay alive...and from my personal experience, even then it's not reasonably possible to defeat him with that setup. He just deals too much damage too often from unstoppable sources for it to work, and potions don't help enough.
I have been able, thus far, to solo every boss in the game with any build (except my summoner due to a lack of real game-development focus on that build type), with no arena preparation whatsoever. It should be possible for all bosses--just significantly more difficult. For the Moon Lord, it now appears impossible to do so, since the most viable option for it was just shattered with a sledgehammer and its pieces thrown in our face for our insolence in using it, while the new armor was pointed at and gestured wildly toward saying "But look! THIS one still works with him! It's a good option now, right?!"
It doesn't exactly help that when the setup currently in place causes one to die to the Moon Lord, it requires them to kill the cultist boss, all four pillar enemy barrages, and the pillars themselves before they can try it again. It gets a bit tedious and loses its challenge the tenth or twelfth time one goes through the motions of eradicating the Cultist and Pillars just to even attempt the Moon Lord fight again.
Even if the new anti-autoheal debuff were toned down, it still wouldn't balance the game--it would just open the option to slaughter him with the Spectre armor back up again, and make it harder to do so. The game doesn't need that kind of 'balancing', it needs something to level out all the classes such that they've all got similar options that pertain to their respective damage types, but are on equal footing.