Might be because I am pretty much familiar with Frankfurt :-).Considering that Frankfurt is a massive hub dealing with legally totally different types of transfer which might or might not involve security, customs and passport, only customs no passports, only passports no customs and quite a few combinations of this or simply nothing but going to the next gate, Frankfurt deals pretty well with this. I find transfers in Frankfurt easier than in any big US airport and certainly easier than in LHR or CDG. Munich is an airport easier to deal with, however there is quite a bit of criticism about Frankfurt just because it is en vogue to bully that airport. Other airports send everybody through security again despite you might depart from the neighboring gate (LHR is a good example for that), Frankfurt tries to make it as easy as the legal prerequisites and the layout allow and this can become complicated.