FMX, from what I have read and experienced with glass, the main issue I have seen is alot of the companies out there do not make the car as a whole, they "cookie cut" (for lack of a better term" the cars and alot of the parts don't line up properly. I know someone is going to say the same about steel, but working with steel in my opinion is a hell of a lot easier than glass. Lining up body panels that are steel is easier than glass at least with me.
But thats what I have found and heard from others, someone with a newer glass car may tell you different, my experience is from the late 80's, so its pretty old info and they have gotten better glass cars today I think