So, I don't think the game is that badly balanced, but the extra mob spawns in the middle of boss fights really do make things near impossible. I got a golem, drakes and wolves with Moder and a bunch of fulings during the Yagluth fight. I was just about coping with the bosses, but half a fuling village chasing me while trying to dodge meteors turned out not to be survivable. If you've just downed a stamina mead to get some extra hits on the boss and you're in the cooldown window for meads and the Bonemass power, you just do not have enough stamina or toughness to handle everything at once.

Don't remember seeing any abominations during Bonemass, just the regular skeletons he spawns. That fight was hard enough anyway.

Are the mobs guaranteed to spawn or is this just the tyranny of RNG? If they're guaranteed, then yes, they're too much and should be toned down. If they're just random spawns for that area, then maybe those should be disabled during boss fights (within a decent radius of their summoning altar).