As one of the people still working in this field, I have come close to stopping developing myself on several occasions since this game was released - the documentation is poor, we are frequently referred back to the FSX SDK. New sections of the flight model are either not described or are lacking in clarity in the description, often not making any sense aerodynamically or even in basic geometry. This has left us guessing a lot of the time, and trying to build known flight characteristics that way is not comfortable - imagine being given the most delicious cake, then you get given all the raw ingredients but only the vaguest instructions of how to put it all together!
Combine that with the fact that the flight model itself has still not stabilised, the continual changes mean a mass of extra work - off the top of my head, I have 6 flight models currently released and each of those needs adjusting and testing before updates can be released, which is time-consuming in itself and as it is not my full-time job, it has to fit in around work and family. This means that I am already pretty much saturated, my time between SUs spent trying to catch up with changes and apply them to what has already been built. There is little to no time to build anything new.
That’s just my personal experience, others may find differently. If it wasn’t for a couple of upcoming aircraft I particularly want to be involved in, there would be one less again!