So, a while back, I asked about this issue with Falco's Up Smash... I found out why.
A thread in the Wii Fit Trainer boards was a research on what moves WFT could crouch under and Falco's Up Smash was listed, so I got curious. So, checking Up Smash's hitbox, the first active hitbox at frame 8 isn't on the ground or anywhere near it. I didn't take a screenshot, but I'll update it later. For now, let's imagine a circle or a dial in Falco's body. Standing, Falco's legs would be at 270 degrees or 6 o'clock. When he uses Up Smash, the first hit involves his right leg at 0 or 360 degrees or 3 o'clock. His left leg is still facing the ground, so his legs form a right angle. 0 degrees or 3 o'clock is about Falco's waist level.
This explains why he can't hit WFT crouching or Luigi going through his Super Jump Punch landing animation. The question obviously is this a major issue? Not really since if you're aiming to hit ground level, Falco has other options like jab, face forward Utilt, angled down Ftilt, Dtilt, Side Smash, Down Smash, falling Nair, Fair's landing hit, frame perfect falling Bair, Dair, Falco Phantasm, and even Fire Bird. That being said, it would be nice if Falco could hit lower with his Up Smash since the animation does show Falco's heel sweeping at a lower level than the actual hitbox, except there's a problem...
Please note I don't know much about game development, so these following ideas might be nonsense. Assuming a hitbox must be called earlier, fixing this might involve speeding up Falco's Up Smash. While it isn't an abusive move or a terribly strong move, people might not like a high active frame and high damage move that is fairly fast getting fast. Granted, this hitbox would be purely for ground coverage, but sometimes, people just see numbers in black and white and might assume Falco's Up Smash is now way too fast. At worst, it might involve making it frame 5, but it could work as a frame 6 or 7 lower hitbox. The other option is "slowing" down Falco's Up Smash. This is something like slowing down the animation making the lower hitbox look like it fits the hit while retaining its 8 frame startup. Kind of like a reverse treatment to his Uair and Fair where the animation was sped up.