Perhaps the problem more lies in your lack of experience with this kit rather than it not being fit for purpose?
A very substantial part of designing a system to be "fit for purpose" is to ensure that it is usable by its target customer base.
A system which is perfectly fine for someone who uses it for hours every week is not necessarily fine for someone who rents a club aircraft once a month and only wants to programme a route into the GPS once a year. These are two different "purposes" and a piece of kit which is fit for one of them is not de facto fit for the other - that's a separate judgement.
When designing user interfaces the professional makes sure he is very clear whether he is aiming at an all-day-every-day user or an occasional user, and will come up with different designs. Or, perhaps, a device with an optional hand-holding mode.
(Of course, back in the real world, if his management won't listen to him, he might have little choice but to end up with something that's not really best designed for either.)
I suppose it could be that there are GPS units which are well designed for the occasional user, and the club I rent from has simply bought and installed the wrong kit. But I've never read any reviews of anything that claims to be designed for the occasional user - I have however read reports of people who, because the UI is so awful, have only ever managed to learn the "go to" button, and blundered through someone else's airspace in consequence.