From an Engie's standpoint I can see the issue though, it's pretty much an instakill for sentries lol. But so is the Direct Hit, although that's used less.
Its only an instakill if all of the rockets hit, which is unlikely because of the 5 degree inaccuracy, which adds up very fast from afar, making it a bit difficult to get a good shot on it.
The Direct hit just needs 2 good hits on an unshielded sentry to destroy it, whether an engie is repairing it or not.
But anyway, there shouldn't be any issue anyway because soldier is one of 3 classes made to destroy nests.