Claiming that GPS is too unreliable for primary navigation but that the NDB/ADF combo is fine seems a bit daft. I've tried to use an NDB as a navaid a few times (IMC training and post-PPL nav) and have never been very satisfied with it - the list of errors is well known, and it's a bit much when you're two miles from BIA and the ADF needle is 90 degrees out. VOR is fine, but I was well into the IMC before I stopped making stupid mistakes with it. That system is OK, but the user (at least in my case) wasn't.
I would have thought that GPS is similar - errors and failures aren't that much of a problem compared to simple finger trouble and lack of understanding. I'd bet for every airspace bust due to some mystical RAIM failure (whatever that is) there have been a dozen or a hundred from people hitting GOTO and going in the wrong direction. IO540 and bose-x may just be unusual in that they understand the box and can work it correctly...
