I'll try to take the hit on this one.
They can be very difficult. Finding exact checklists even from the factory are difficult, and if your airfiles (if) do not match performance, then the builder/developer can really make himself look bad.
But thats not always why they do not have checklists and such.
Some people cannot find them and one will be in the package and they do not seem to find it.
But I must say, making a Mesh model is very difficult. Making a good airfile for it so it handles awesome is another great wonder to conquere. Creating photo real textures is the art of a person born with the talents of Michaelangelo, able to put in all sorts of detail that you could never begine to imagine (dusty, scratched surfaces, mud, rivets, wavey metal from fatique, etc, etc, etc). Then you have gauges! This is like knowing how to write in an alien language, that is based in another dimension... . These guys know how to write it fluently and can bongle your mind on how they can accomplish creating elaborate mechnisms that are not supposed to work in FS to begin with.
Lastly, after making all of that, fitting it into one package, putting in an installer that can find your FS hard drive on a remote link in another office on a seperate floor from yours, and then have a good checklist that will match with yours... is... is... IS.....!!!!!!
AAAAAAAAAAAAAAAAAACK!
<--- takes a deep breath
Many developers do not put in good checklists, but some do. In the past, there have been people that saw this as a need, and they quickly filled the void by creating their own checklists, doing the research, (which can be extensive) and made a very cool downloadable set that others could enjoy. One gentleman did some that had pictures and everything. Some cool work he did. He might still be in the biz.
That can be why...
Bill