Falcon's dair OoS can beat nair>shine, but decent spacies will catch up quickly and nair>utilt, which will open you up just as bad as him getting stomped. And so the mixup is to keep your shield up... not ideal vs spacies. I recommend escaping pressure asap every time, rather than putting yourself at risk fishing for an opening. Just shield at unexpected time and space (dash>shield-stop, pivot-shield etc) and shieldgrab unsafe aerials.
j3ly
does any falcon actually uses nair OoS to counter attack ? Seems like CC bait to me. Maybe if you do a shield stand-off vs sheik and she freaks out and nairs, you can start an air wobble with sh nair. But I love instant uair recently, it's just that the hitbox is tricky to find on grounded opponents (especially low profile chars).
Also since it wasn't mentioned here : reverse upB OoS ! It's so funny, always catch people off-guard, and can get you out of hairy situations. And it covers your back, which is nice.