I can't speak to mobile or flash since I've barely even dabbled in those, but when it comes to desktop if you have big dreams and put the work in you can definitely make real money.
As long as the game you want to make is possible to make in stencyl, (stencyl primarily is for traditional 2D style games, although I'm sure more clever people have figured out ways to make less traditional games, core 2D is really what it's for) then the only limit to achieving success is your own ability to design and create it.
Speaking for myself and my "Metroidvania", I have a desktop game that runs at 900p (I'll do 1080p next time), works in full screen mode on my TV and looks good, it uses controllers, people can map their own controls themselves, you can save games to multiple slots, data can be read and written from arbitrary text files (using
this beautiful extension) it's facilitated and launched by Steam just like any other game (currently not publicly for sale though) and the game takes place on a large map comprised of hundreds of scenes. The game uses full screen shaders to tweak and enhance its appearance. In the future (possibly near future) as Open FL unrolls new versions, stuff like single target shaders and more blending modes for stand alone and other great stuff will become possible in Stencyl. Console exports may even be possible...
It's really a childhood dream come true to be able to make this and not even be a legitimate programmer. When I first started Stencyl I really knew absolutely nothing of programming concepts, and I'm completely amazed by what I've been able to do and I'm still not even a real programmer. I routinely frighten Justin with my ignorance.