So on my ranger playthrough with this mod plus many others, I find the bosses to be overly difficult for when you're supposed to fight them. I find the main issue is that they fire way too many projectiles, giving you little room to avoid them and they hit for very high amounts of damage, at least on Expert. It's like you can maybe take 3-4 hits at most before you die. Considering how easy it is to get hit, I don't enjoy fighting the bosses. When compared to other vanilla bosses around the same progression and even some other modded bosses like Thorium's, those are much more fair fights to me. Terraria isn't designed as a bullet hell game, although that's not to say you can't make projectile spam bosses. You don't have the instant acceleration with a tiny hitbox that makes bullet hell games work normally.
The Etherial bosses were even more over the top. On top of even more projectile spam, one hit can deal hundreds of damage. It's like you're not allowed to make any mistakes against the insane amount of projectiles some bosses gain. I'm not even sure how you're supposed to beat some of the bosses like the Queen Bee having super bees that seem to act as meat shields against your homing weapons. In particular, Plantera fires so many projectiles that I have no idea how you're supposed to avoid all of them, especially when each of them hits for hundreds of damage.
It's not that I can't handle a challenge, but I personally feel this might be a bit much. It's not that I don't have mobility options either or some arena setup either. I saw a video of a no hit run against the bosses (although not Etherial bosses), but that's not going to be the norm from the majority of players. Also I don't know how many attempts it took in the video to record a perfect run per boss. It's like the only real chance I have are using the weapons made from Bysmal Bars, which deal extra damage in the Etherial. However, even at reduced damage outside the Etherial, I think they're OP considering when you can get them, at least that was my experience with the ranged weapons.
I do like the new items and even class offered by this mod, including the Etherial dimension. But I dread the bosses.