I'm fine with it on high level or high CR monsters. Beings so strong that aspects of of them are unavoidable if not immune.So, what are peoples' thoughts on auto-damage?
For myself, I don't care for it. IMO everything should have a chance to not-effect a PC or the PC should be able to mitigate it (such as the half damage saves).
The creatures mentioned upthread are ones I've never used or encounter, personally, otherwise I would have broached this subject earlier.
As for spells like magic missile, I am not fond of that aspect of it, but at least it is (typically) a limited resource instead of unlimited.
Like a Marut. It's a Inevitable and being of pure law. There is no random chance with its slam. It will attack perfectly every time. A marut will not miss. It's up to you to withstand or dodge the blow.