Bosses do not have big curses; in fact, the only boss I remember having a curse is a queen bee, which is also the least tanky boss in the game. And she does harmless small curses that aren't that bad.
Monsters with curses are indeed annoying, and while the most recent patch moved their curse move onto the second turn instead of throwing them at you right away, there is still some leftovers that aren't good. And adding a curse to the first turn Salamander in the swamp in my opinion was the worst change in this patch:)
It isn't as unplayable as you describing it though; first 9 floors that are considered the actual game (and the rest isn't supposed to be very balanced as its called endless mode and will outscale you anyway) can be beaten on a lot of setups and curses are only bad if you deal very low damage, dont have any sources of life recovery and have your entire inventory occupied with useful items, none of which you can discard. It also should be all at once, because otherwise you can either kill enemies quick enough, outheal the damage taken instead of curse (yes thats an option and it is specially good against big curses) or just take a curse and use it right away if you can neglect its effect on use. I'm not even mention a legendary that destroys adjacent curses since it also require a bit of healing and some luck to find. But overall, curses aren't that bad, specially for the 9 floors of the current game.