Playing campaign, got to the Crystal Nexus. It's fun, it's weird, things get mutated, yes, that's neat and all. Get to the final bossfight, the Tethervore. It enters with the following stats and abilities/mutations: 4Atk, 16HP, Lifesteal, Wither(? don't remember the number, may have been 3), Relentless, Damage Cap 2, and might have had extra HP, I don't remember. As always, summons a random mutated enemy every turn.
Do you see the problem? Can't deal more than 2 damage per attack, boss will always heal four until I have shields. Can't stun to stop lifesteal, constantly losing die faces to Wither, and rapidly losing ground if I do focus attacks against other enemies that are constantly spawning. Over time I could have potentially won, seeing as I was playing Demonologist and had a number of Night faces, but the turn timer made it impossible. Hell, mathematically, if I did exactly 8 damage to it every turn, starting from turn one, it still would have taken 3 turns with perfect rolling luck on all four dice, each turn, with the constant hope that I don't die to whatever else is showing up in the meantime.
So, here's my suggestion: code certain mutation abilities to be impossible to spawn onto a boss. For example, Lifesteal+Damage Cap? Disallow Relentless. Relentless and Lifesteal? Disallow Damage Cap. Etc... Basically, there are combinations that make it next to impossible to succeed, and in a campaign setting on the next to last level? It was devastating.