About the healing, first, it's a matter of view, second and most importantly, I isn't a big issue, for me, first with Eye of Cthulhu, I tried to do so the healing isn't much (tell me if the healing is too much) but to make a bigger punition when the player is hit, and when it eats the servants, it's to encourage the player to destroy them all, then for the Queen Bee, it's only when a bee has lived long enough, but you should've killed them before, for The Destroyer, it's only the probe's damage, so if you destroy them effectively, it wouldn't be such an issue, but even without doing so, if you dodge effectively it isn't an issue too, but even without doing so both, the proes don't deal terrible damage (except melee, but, y'know, expert mode), and also The Destroyer is very easy to cheese, so it needed it in some way,
and also it's good to hear an opposed point of view,
and also what do you think about the three Mech Bosses appearing at the same time ?