The info about GUR in your post is not actually correct. If the ball is behind a tree or in a bush within the GUR, the bush or tree is part of the GUR and relief is given. Let’s say your ball is on a hole caused by sprinkler damage and it is marked GUR. You would have no reasonable play from there but you would get relief because the entire area is GUR.
The other examples of “ no relief” are correct. If your stance is on a cart path but the ball is unplayable beneath a bush, no relief is given. Another…your ball is behind a tree with no way to make a stroke, but you are standing on an ant hill… no relief is given.
Although GUR is dealt with in Rule 16 as abnormal ground conditions, you should check the Definition of GUR and read the Interpretation that goes along with it.
One oddity on the rules (among many) concerns LCP compared to embedded ball relief. Some tournaments will give LCP anywhere in the General Area, and there is no restriction on its use. Even if you have no swing at the ball, you are entitled to LCP relief. If your ball is embedded, though, you are NOT given relief if you otherwise could not play the ball where it is embedded. Strange but true😀🏌🏿♂️
As you say, these are issues that the average player is either not going to know or will think he knows. That is why the cell number of the head rules guy should be given to every player.