Schools don't tell you lots of things, in the interest of revenue.
The #1 thing they don't tell you is just how restrictive self fly hire is. OK, you have a brain and should realise it, but the reality is that most students don't grasp it until they try to take the plane away for a few days.
The #2 thing they don't tell you is that most of the junk most of them have available for hire is unsuitable (and often illegal) for IFR flight.
As regards not being able to use non-FM immune kit in Class G, the situation is not clear cut.
You also still have the following navaids which are not affected by FM immunity requirements, and none of them is prohibited or in any way restricted from being
used, at any stage of a flight:
ADF
DME
GPS
The crucial thing is that the regs specify what must be
carried, not what may or may not be
used.
Take a guess which of the above devices is the one chosen (as the main one, with the others used as a backup) by just about all pilots that fly seriously.