I have no opinion on ULA+NPTv6, other than Experimental doesn't mean you shouldn't use it. How else would people experiment. It does mean that the level of vetting by the IETF is potentially lower.

WRT IPv4 NAT, I'm not sure how much we can infer from the status. Many people at IETF were (and some still are) very anti-NAT, in part because they felt that IPv6 was the right solution. As a result, the IETF really avoided doing anything that looked like it was endorsing NAT, even though it's obviously just a fact of the Internet.

My general point is that the category or track of an RFC may mean different things to different people (assuming they're even aware of them at all).