Not any different from C - a given C compiler + platform will behave completetly deterministically and you can test the output and see what it does, regardless of UB or not.
Sure[1], but that doesn't mean it's safe to publish that C code - the next version of that same compiler on that same platform might do something very different. With machine code (especially x86, with its very friendly memory model) that's unlikely.
(There are cases like unused instructions becoming used in never revisions of a processor - but you wouldn't be using those unused instructions in the first place. Whereas it's extremely common to have C code that looks like it's doing something useful, and is doing that useful thing when compiled with a particular compiler, but is nevertheless undefined behaviour that will do something different in a future version)
[1] Build nondeterminism does exist, but it's not my main concern
To fix bugs, sure. They don't generally get updates that contain new optimizations that radically break existing machine code, justifying this by saying that the existing code violated some spec.
Maybe your program worked due to the bug they fixed.
RDRAND always returning all-FF is exactly the kind of thing that's an obvious bug, not a plausible-but-wrong result.