I noticed that FP10 breaks "You have to burn the rope", but otherwise, I haven't run into any problems with it.
However, I also haven't run into many things that need it. Off the top of my head, there's the Alchemy-based Flash port of Doom and maybe one other game. I recall seeing that only around 30% of people are using FP10 right now, so it doesn't seem worth it for developers to target it right now.
