Purpose of UUID is to be unique, not random. There is no need to wrap RDRAND into another random generator. One can use time/counter as one part of UUID and kernel provided PRNG, e. g. getrandom(2) for another part.
During early boot, the kernel will either block indefinitely and/or log the uninitialized urandom read as a security problem. It was the entire reason that brought systemd's rdrand code into existence.
