I've used FDT for a couple of small apps. There's lots of general borkyness, but as far as I can determine, most of it stems from the fact that the Facebook APIs are a highspeed moving target and are poorly documented (this was especially true around the introduction of the new profile design). I don't think one should fault the FDT developers for lack of compatitability when this is probably mostly due to facebook retirering or changing core features.
(I'm not affiliated with FDT although I have suggested a few patches).