When you find out, I'd like to know! It's pretty laggy if you get caught near the ground at the end of it, so when applying shield pressure it may not be worth it, since they can grab you in your lag frames--That is, unless you try to apply pressure and short hop to their backs and they don't pivot you. It also doesn't take priority over anything I've faced against. Maybe it'll be a good move to wait people out for during an air dodge on a low ceiling, as it may send people higher than your up air? (Similar to Luigi/Doc's down+B?).
I hardly ever use this move because of how risky it is to use it, but I would like to use it more so I can be less predictable!
Edit: Upon testing in training mode, using Mario's uptilt on an opponent (Zelda in this case--Random CPU), and following up with a Dair KOs at around 112%-115%, which is fairly early for any of Mario's attacks, as the only attack that will kill below that is is F smash. You could also carry them upwards should they air dodge after an up tilt, and you wait them out a bit...It doesn't combo well if they have good DI, but it's definitely possible! As a quick note of reference, this is significantly earlier than his Uair would kill, as that won't KO until around 140% or even 150%...If not more on some heavyweights!