I usually transfer from non-Schengen to non-Schengen and as long as I follow the signs from one gate to the other I don't have problems. If I have the time to go to a lounge it gets more complicated because they never seem to be in the place indicated on the map.

Actually the worst thing I find is there isn't a proper map either on the LH or FRA website. Many other major airports have detailed maps showing lounges and even individual shops and gates. I can't understand why FRA doesn't and didn't have one before the current construction work started.