I just want to bring up, if you notice that Falcon's knee always seems to A) Miss or B) Non-sweetspot when you attempt to hit your opponent after doing a SHFFed u-air/raptor boost/run off a platform, then this might help you.
I have noticed that if I fast fall both before and after doing the knee(as in, you use it, you're extremely close to your enemy, but you still aren't close enough to hit with the hitbox), you will most likely hit them whether you shorthopped or full-hopped your knee. If you seem to be missing from above, fast fall it, keep missing, short hop it. If you miss from below, stop short-hopping/fast falling it. Did atleast one of these combinations work for you? I'm ABSOLUTELY certain that it is the case.
What I'm trying to emphasize, there are different knees for every situation. The reason that people may not hit as often with THE KNEE, but complain about hitting with his forward air(don't even dare call the knee and falcon's fair in the same category, they are too different for that, it's too good for that), is most likely NOT timing(unless you plain out suck at falcon), but lack of correct application for the knee.
Yet there are times when you may want to hit with the fair instead of the knee, there are often different ways to insert both into your strategy. SHFFed spammed fairs MAY let you knock off your opponent and off the edge of, for this example, Battlefield, at which point you can run off the edge yourself and knee them into oblivion, or stomp them with your down air then proceed to recover(NOTE: stomping them will most likely kill you unless you are right next to the ledge[some exceptions]) and tell them to "Show me ya MOVES!" or "COME ON!"
Point is, different application of each is important(I have tried SOME of what I mentioned with a decent sucess rate against a HUMAN, not a computer), both knees are to be appreciated for what they can do.
