I've come across this a couple of times. That flash game development is not 'proper' game development when compared to XNA or even Unity. Mentioned here: Need guidelines for studying Game Development

Also here in some comments : Where to start with game development?

This judgement also befalls java, according to some. Is it because in flash its so easy to draw graphics and to import and add on to the stage any element we want and also because flash needs a 'container program' to run and others don't?

But flash is by far way easier to 'distribute' than any other of those mentioned above. Maybe except for iphone or android games.