This was a good article. Bits of entropy are not lost as the CSPRNG spits out bits 1:1. Nobody thinks that a 64 bit key is compromised after a CSPRNG outputs 64 bits.

However RDRAND should not be trusted because we know for certain that motherboard BIOSes have the capability to force the output of RDRAND to -1 every time it is called. Hardware vendors have demonstrated this to us as a "bug" multiple times, but I consider it a "wink". They have fixed it with BIOS patches. The fact that this is patchable at all indicates RDRAND is not secure. The BIOS can control this behavior; therefore it is IMHO very highly suspect and not worthy of trust. There is no legitimate reason why a ring oscilator circuit with it's whitening/etc should ever output a predictable sequence of any kind.