While I gave thick brush or jungle as an example of complex terrain, i of course didn't mean those are the only types of conditions that can attenuate the signal.
The point I'm getting at is, as good as GPS is, and I agree it is an incredibly useful instrument, users need to understand its limitations
The GPS can be tracking only 3 satellites, and while this will give a very useful approximate fixation, this might continue for some time until it picks up a fourth satelite. During that time there can be some drift. Should the GPS be coupled to navigation system, and if the user had entered a location into the nav aid earlier that was not that accurate, then it is possible the nav aid will try to incorporate that difference from where it thought it was to what the user put in, exacerbating the situation.
Since GPS relies on its latitude for its direction, having a grid that is somewhat skewed can degrade any bearings displayed, though for most navigation purposes its not that noticeable.
But these things combined (bad fix and orientation) can become very noticeable if for example lasing a tgt using a system that relies on inertial/GPS navigation (and why devices like PSS SOF can help resolve them)
Another reason I believe map and compass training is vital, is from a human factors point of view. The DAGR currently in use, when used alone, has the draw back of displaying primarily just a location, altitude, and direction/heading, whereas most civilian applications use a more effective rolling map type of display to convey surrounding information. I think for that reason alone, sound knowledge on how to use a map helps a great deal in maintaining situational awareness.