You lose buildpower when canceling so that's already a pretty major disadvantage, regardless I think it would be fair to only give back the same % as a wreck would.
I don't really see any disadvantages to this, it's mostly a quality of life improvement. The cancel all but current order was a very nice feature, but there's a few cases where you'd want to cancel immediately like indeed if there's a bomber threat and you want AA asap. Or sometimes when you have won control over an island and don't need any more land units, so you want to ctrlk that factory but dont want to do it before finishing production so you dont waste mass.
Getting the wreck of an unfinished unit just makes perfect sense.