Fair gives a fair amount of shieldstun and shield pushback, turnip has pretty good shieldstun as well. I'm not really sure if you can punish it with shieldgrab, perhaps characters with fast and ranged grabs could. You can also roll after Fair in order to avoid the turnip and you might still be able to punish her if you have a fast roll. However, all of this assumes that Peach does throw the turnip. If you expect them to react to your throw by waiting in their shield, you can simply run away instead of throwing the turnip and get away safely. If they roll, you bait it and punish. All of these things give her options to get away with Fair on shield instead of just being unsafe and getting punished.
It's also safer on whiff if you whiff it right before their feet since you have got your turnip to cover you. All in all this is a pretty powerful tool so I to really wish the hitstun thing was just never discovered since there is a chance they might patch this, how small or big it may be.
If I've noticed two things while playing this game by now it's that shield stun is nearly non-existant to the point it might as well not exist, and that most character have rediculously godlike rolls. But maybe I should go back in and check fair in particular, because you have a good point with the push back it has on shields.
. I am however pretty sure that you can very easily shield the turnip (or the fair if they dont throw it), drop the shield, and dash in to grab with how little shield stun there is, and how much lag the throw animation has.
You can punish (some, but definitely not a lot of them from what I've noticed) rolls with it I believe, but what I'm arguing is that eventually there is no reason to roll when you block a fair. Not throwing the turnip isn't really an option, because fair doesnt auto-cancel untill very very late. Unless you stay floating for long ofcourse, but then you wouldn't even need the turnip cancel.
What, to me, it comes down to is that the opponent definitely has enough time to see all these factors and come to a solution that pretty much solves it.
What makes this tech amazing though, to me, is the mixup opportunity after landing the fair -> downB turnip throw combo. And you're definitely 100% right that this tech is godlike in case you whiff. I might even say that (until everyone knows about this and expects it) that is one of its best uses, people will immediately try to whiff punish when they see the fair, and then BONK turnip to the face. But people will eventually learn not to try and whiff punish if peach has a turnip, or try and punish the turnip throw which would be difficult for some characters, but definitely possible.