Haven't used MMF, but I have Game Maker, and as Lucid said GM isn't even in the same class.
If I were to continue the comparison in metaphor I would say GM is a program with a front end like a bike with training wheels, and the back end like an eighteen wheeler.
Obviously a huge learning curve in between.
On the other side of that I would compare Construct to a hot rod in the making, put together by a couple of shade tree mechanics. Now in this case the front end, is just about as powerful as the back end, even though its pretty much street legal, it still needs a lot of work.
Both programs can have seemingly random crashes, like just about any other program out there.
But unlike most closed software out there, if you can identify the problem, there's a good chance it will get fixed in Construct.