RDRAND is a microcoded instruction. The chipmaker does not publish that code, but they absolutely control what it does. If it has any flaws, whether deliberate

RDRAND is a microcoded instruction. The chipmaker does not publish that code, but they absolutely control what it does. If it has any flaws, whether deliberate or accidental, you can't fix them (the processor vendor might be able to fix it with a microcode update, but no one else can). That's why it isn't used. Your soft PRNG is an algorithm and a relatively simple one at that. You can test it, and verify that you get exactly the same sequence for a given seed whether you use a processor from Intel, AMD, ARM or someone else. Trying to detect and back door it would probably break a lot of other code. The bad guys would probably choose to attack you in a different way.