Enemy HP has been substantially increased to compensate for their reduced AC, so those won't do as much damage as is expected.
This actually isn't that much of a thing. The only enemies that exhibit this behavior are the goblins. Almost everything else is close to their actual tabletop stats. The bosses however are generally agreed upon to be extremely overtuned stat-wise if you're just fighting them from the perspective of a DnD game and not abusing Larian mechanics.
The real problem is generally
everything else - the sheer number of enemies in some encounters which makes the action economy heavily stacked against the player, the lack of key reactions like Shield which would allow Wizards to defend themselves during the enemy turn, battlefield design which makes height such an overwhelming factor, the existence of the height mechanics themselves that results in the idea that AC below 14 might as well not exist on a statistically practical level if you're being attacked from high ground, enemies having new and overtuned items and abilities such as the special arrows and teleporting spiders with AoE spits and the minotaur being able to jump at your party from way beyond sight range with an AoE shockwave attack
as a bonus action, and so on.
All that combined makes casters feel iffy to utilize if you're not using them to set up some insane alpha strike tactic.
