The take away from this thread is that nobody with a Nexus 6P on Project Fi has gotten the OTA update.
I've seen a rumor in the Android blogs that there was some sort of issue uncovered with T-Mobile, so T-Mobile was removed from the eligibility list. Since Fi uses T-Mobile, it would of course be subject to the same issue. A later version of the rumor includes US Cellular as being affected. Nothing I've seen talks about what the issue is, just that there is one.