It's an interesting mechanic for sure and can potentially mess your opponent's plan up, but IMO the biggest thing that makes transform less useful is the fact that the creature that was transformed goes into the void. This means transforming creatures like Hippacria's Monster won't stop your opponent from reviving it with Neferu's Khopesh and the like. Would definitely be worth looking at again if the behaviour was changed such that the creature that was transformed doesn't go to the void.