Won't talk much about the FKK since there are plenty of review already done. My advice would just be to figure out the transportation since some of them really can be quite far, and I got lost walking to FKK shark
For people in Frankfurt, avoid the RLD. Looked like a dump to me, and while the price was attractive it was just depressing as hell walking around.
Also there seems to be alot of Asian bros there. I think some of them target asians cuz some of the richer asians are willing to pay higher price. So if you go there please don't spoil market!