The issue with X-Plane is it has always been a moving target, with each point release adding new features and possibly breaking old ones, that why you don't see much commercial developers on it.
Another problem for scenery developers is that a certain scenery area has an "all or nothing" approach, instead of a "layered" approach like MS Flight Sim. This means, for example, you couldn't use a photoreal scenery made by developer A, together with an airport made by developer B, if they cover the same area.
This obviously helps a lot with optimization, the less flexible an engine is, the easiest is to make it faster, since you trade ease of expansion with performances.
And, last time I've checked the SDK, many things we deem crucial to replicate what we usually do in FSX, were still missing.