The point is much of what the C standard currently calls undefined behavior should instead be either unspecified or implementation-defined. This includes the controversial ones like strict aliasing and signed overflow.

Additionally, part of the problem is compiler devs insisting on code transforms that are unsound in the presence of undecidable UB, without giving the programmer sufficiently fine control over such transforms (at best we have a few command line flags for some of them, worst case you'd need to disable all optimizations including the non-problematic ones.)