One big difference between unsafe Rust and C is that C compilers have flags to turn off the UB, so you have a lot less mental load when writing it. You go from e.g. "if this index calculation overflows, we may read from outside the array, because the bounds check was deleted" to "if this index calculation overflows, we may read from the wrong index, but never outside of the array bounds". UB is Damocles's sword and the speed gains are usually not worth it. With UB your program can enter a buggy state that you cannot detect, because "it cannot happen". Without UB, your program can still enter a buggy state, but you can detect it and potentially recover or crash immediately before even more things go wrong.