Usmash out of shield might work if your opponent isn't spacing properly. I prefer to use it as an interception tool rather than an punishing tool, though. Utilt is faster, and harder to SDI. Uair is better if your opponent is tall enough, because it's also very hard to punish.
The problem with Flip Jump out of shield is that it has a predictable trajectory, and you can only change that trajectory by kicking, which barely has an effect. Honestly, the risks outweigh the benefits in this situation.
The reason not many ZSSes usmash is because there are very few situations where it outclasses other moves. I tend to use it primarily, as I said, as an interception tool. I also use it if my opponent is on the platform above me, especially for platform tech chasing. Most people only like it for the latter of the two.
The real issue is that if your opponent SDIs out, it's very punishable.