It's funny, because they already have debuffs that punish failure without outright killing the target. The generic damage downs. Paralysis. Sleep. Silence. Pacify. Mini. Stun. Any one of these punishes your output pretty severely without causing death.

With respect to Ultimates, I've healed UWU. It does have some scary heal checks...the first few times through until you figure them out. And not very many of them. In my experience, you have a few pockets of intense healing maybe once a minute or every two minutes optimistically. I pulled grey damage numbers on my last clear because I wanted to make damn sure we hit the end of the fight and it got sloppy. We cleared. With grey damage output, you know what I spent about 70% of my time doing? Stone stone stone stone stone stone stone stone stone.