[nulld3v]

The problem would not be that it is commonly used, the problem would be that it is abused. And I don't see that happening currently.

The assertion that "If a library panics on user data, that's a pretty serious bug" remains true.

If a library is panicking on invalid user data, it is because they are abusing panic, which is a serious bug. Or they just didn't realize that their code could panic, which is also a serious bug.