I have been using OpenFL since some years now. And have successfully ported many applications from Flash to HTML5.
Till now, I have only used Movieclips as bitmaps. The well-known problem with bitmaps is that applications take more time to load, size cannot be scaled ( unless the assets are resized to 2x the original size, but that will increase the size, and affect the loading time again), and chances are that the bitmap may not look as sharp as the vector ones. The heavy animations can skip some frames because of the bitmaps, and eat CPU while running.
I sometimes see some issues with fonts not rendering properly, and sometimes the server shows output from an old version when compiling or debugging. This part eats time, and only solution is to delete the exported files manually, and restart the IDE, and refresh the browser cache, every time it happens.
I remember, some problem with masks. Also, filters used on Movieclips must be rasterized. They don’t export from flash.
I am too looking forward to use vector for some of my OpenFL project. I see snap SVG Animator with Animate IDE is one option, but I am not sure how to use SVG with canvas. The snap-svg api is having a learning curve too.