The biggest reason to not use the Unreal engine is that it hurts your bottom line actually.
It's rumored to cost over a million PLUS you have to pay epic a percentage of the games intake.
Unreal should only be used for your midstream/games you expect to be ok or are worried about sales.... as it will absolutley kill you if you plan to make a big profit off it.
Hence why few teams have probably used it more then once.
If your going to make a bunch of the same series... your way better off making your own engine.