Are you sure about this? I feel like my instant uairs when shffled really do combo better than an AC. I know the frame data says it auto cancels at frame 22, but I think that means that it ACs if you land after frame 22, not that you land at frame 22. Have I been misinterpreting that thread all along?
In the frame data post it's written as 22>. In plain English I read that as 22 or greater than. Otherwise I imagine it would say >22 (greater than 22). I tested it a bit in TAS and it seems to be that if you land on the 22nd frame then it auto cancels. That actually means your short hop can be 21 frames long and it will auto cancel (so my original post was err'd)
Either way, the absolute difference between the two is negligible. However, the likelihood that you can do frame perfect 21frame short hop, instant Uairs seems tiny to me (don't forget that a full, non fast falled short hop is 31 frames). So depending on the auto cancel is impractical. If you unintentionally do a 20 frame short hop and don't l-cancel, you're stuck with all 14 frames of landing lag. And short hops you do that are greater than 23 frames add to wasted time.
Remember that this all assumes you're doing perfect instant Uairs as well, meaning the animation starts on the very first frame you're the air. Any wasted frames you have in the beginning will also add to the likelihood that you won't auto cancel within your fast falled shorthop.
That's why I think it's just better to go for consistency and SHFFL it. At the very least you should always hedge your bets and L-cancel the Uair. If it auto cancels, there's no penalty for the L-cancel input.