To provide some more context/motivation for why you might care, I write safety-critical code. I'm often advising people what they need to do for certification, etc. If all you need to do is ensure that you never execute undefined operations and knock out the list of specified UB, that's totally, 100% manageable. Throw some sanitizers on, provide realistic input, and test the hell out of it. Normal stuff.

If the reality is that any UB can invalidate the entire program (as is the interpretation taken by other standards re: C), then that's not remotely sufficient. You have to ensure the complete absence of UB.