Giovanni, HAHA, well, I didn't say the Flentrop was my overall favorite, but it was my favorite practice organ at Westminster Choir College. I like a practice organ that is sensitive and shows every flaw in technique. It is easy, then, to transfer over to another instrument, including non-trackers. It doesn't work as well the other way around, however.
Actually, one of the things I love so much about the Tannenberg in NC is its sensitivity, especially its flexible winding system. It's probably the most sensitive winding system I've ever played, and for sure, if you are not sensitive to it, the instrument can sound pretty bad. However, played sensitively, it sounds like there are individual instruments playing. Trios sound fantastic on it!