@Paul oh no no, not human transformations. Even part 1 genin could tell them apart. For instance, in the Zabuza arc, Naruto threw two shuriken at two Guard Ninja. They deflected it and then the shuriken turned into Naruto, and he attacked from behind.
Do you remember how Minato bested Obito with his FTG? If someone threw a shuriken at an opponent, and then transformed the second they dodged it, it would almost be like having a watered down Flying Raijin level 2 lol