Other than the disappointing lack of new AI, most of the bosses are actually pretty fun imo. If you have made at least decent preparations for the fight, the difficulty feels just right, even if many hardmode bosses have the capability to kill you in 2-3 hits. They are bosses after all and should accordingly deal very high damage.
However, bosses 2-shotting you is one thing, but giving the same absurdly high damage numbers to normal enemies as well just makes things frustrating. In a boss fight, and especially in Terraria, you are given a lot of control over the fight, from the arena to the preparations to the location and time of the fight. If you lose, you can try again when you feel like you are ready and better prepared.
You decide when, where and how the fight takes place, which allows you to counteract some of the boss' difficulty that their massively buffed stats bring to the table.
You just don't get that same feeling when an early hardmode Pirate Invasion rolls in and turns your base into a graveyard because each enemy deals roughly 100-200 damage per hit. You have very little to no time to prepare and you're forced to sit through all of it, watching your character get hopelessly pounded into the ground over and over again. And all of that just because of some dumb RNG.
Now, you'd think the Lunar Events wouldn't be as bad as the above example, since you actually get to decide when they happen, but no, they're arguably even more of a pain. Nebula Floaters are now officially the worst enemies in the game and Crawltipedes have a chance of actually one-shotting you if they catch you lifting your
more than 30 cm off the ground because you're trying to evade the other 50 million enemies nibbling at your feet, except it's not really nibbling, but more like getting your face caved in with the force of a dump truck reaching terminal velocity. At least bosses have set attack patterns that you can memorise and dodge accordingly, so the damage is somewhat justified, but not if it's an uncoordinated mosh pit of pain where even the tiniest projectile from an off-screen enemy can easily deal 300+ damage if it does as little as graze your left pinky toe.
TL;DR: Boss difficulty, while no AI changes, is pretty alright. Event difficulty is lunacy, both literally and metaphorically.
Conclusion: If you're only in it for the difficulty and not for the new drops, most of which are just cosmetic items anyway, you might be better off just playing Expert + for the worthy seed. Haven't tried it personally, but it already seems a lot more promising.
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