MJ
Interesting - are the KLN90 and the Trimble really installed in an airliner? Surely you must have triple inertial nav? I recall looking at a used SEP ~ 2yrs ago which had a KLN90B and I checked it out then; discovered the design was about 8 years old back then. That's a very long time in GPS receivers.
I don't particularly disagree with anything you say. However NOTHING is 100% reliable. Everybody should know that. It is widely assumed that one's brain and the "Mk1 eyeball" (to use the favourite expression of the anti-GPS crowd) are 100% reliable, but they aren't. GPS is a lot more reliable than any other means of en-route nav.
As you suggest, the "IFR approved" units (which were generally intended at the American PPL/IR market which is almost nonexistent in Europe) are not suitable for the average PPL flyer because their moving maps are too small or non-existent. That's why I think fitting a GNS430/KLNxx in a VFR plane is pointless. A much better solution is a GPS with a large MFD or, if you want to save a lot of money and don't have stormscope/radar data to show, a large moving-map GPS like the KMD150. This is a modern product which, with a rooftop aerial, will be highly reliable in reception. Garmin are not a player in that market, for some reason.
You say "Great but no one explains to them the importance of ..." Perhaps this is the real "GPS drawback". Officially, nobody likes it, so nobody is going to tell people about limitations.