Not every feature of Flash is supported in OpenFL, but many are. I’m not sure how many the Flex classes may be relying upon, but text (like stylesheet support) is not supported, nor is the TextLayoutFramework or certain features like accessibility options.
There may be a few things that Flex could rely on heavily (like tab order) that may not be quite perfect (since most games don’t rely on features like that) but issues like that would be simple to fix, and I would love to help support those better.
Is there a “Hello World” Flex application you could test on this process? That might be a helpful step
Also, it’s worth considering to go from AS/MXML to Haxe, but compiling to Flash Player. It’s highly valuable as an inbetween step, so you can be sure that things converted as you expected, without changing the runtime implementation. Then we could support additional targets, are you going for HTML5, or native?