Kai, what hops did you use for FWH in that pilsner that turned out so well?
Me thinks this is like dry hopping, it works better with some hop varieties than others. And as you said, there are likely a number of factors in play. I'd suggest looking at water chemistry, hummulene/myrcene/etc. content of the hops, and of course malts used (for balance). The three times I used FWH and it turned out nice, was with 2% spalt. Four times I did FWH and the results were not good at all; the hops were hersbrucker and tettnang in those cases, at 4.5+%. It would be nice to know more about this as when it works, it's great. At this point however, I'll probably only try it with low aa% because I don't like that lingering bitterness and it's not worth risking a beer.
This subject is probably worthy of a phd!