It is simple - in order to fly under a 61.75 once issued, you need the originating licence to be working i.e. thst you can fly in the originating country using only that licence. So it must be accompanied by a current medical, a current flight review or the originating country's version and you must be current on flight experience (hours, take off & landing).
Then with your 61.75 you can fly on FAA with only the following stipulations, you must have a current FAA flight review and be current on take off and landings for an SEP. In addition you must have sign-offs from an instructor for complex, tailwheel. The 61.75 does not give you ratings your original licence dies not e.g. FAA PPL has an automatic "night rating" the 61.75 does not unless you have such a rating on your originating licence.
The idea that the 61.75 allows one to skirt around one's originating licence is incorrect. For example, one cannot get a 61.75 to enable legal flight by getting a Class3 FAA medical because one has been denied a Class 2 in, say, the UK. The 61.75 does invalid once the originating licence does when one is denied the originating country's medical.
Anyone who says different is simply wrong.