Shago with a flipout would be way, way, way too strong.
The reason his grounded mixups are allowed to be this dirty is because a lot of important ones cause a launch, and he needs meter to cash them out, which he conveniently also needs to keep his best mixup tools safe-ish. The idea is that it’s supposed to be a constant decision making process for him which thing he prefers, either safety on mixups or damage cashouts.
If you give him the ability to surprise reset you at any time during his juggles, then you’ve a) made him much harder to break and b) done the Thunder thing to him, where he will now have a ton of meter to use for tons and tons of surge moves in neutral, or whatever else he wants, because he doesn’t have to commit to shadow cashouts to make his offense potent. Right now, if you get launched from Shago and he doesn’t have bar, you know you’re not in for too much damage and you aren’t about to get hit by the 5-way mixup, but with a flipout that will never be the case.
It’s not just about “let’s give him a flipout because that’s a mixup tool and Shago is a mixup character”, it’s about the overall tone and gameplan they want him to implement, and that does not (and cannot) involve flipout.