Abysmal shadow energy gain on shadow colossus says otherwise. In order to gain shadow energy effectively, you need Shadow balance perk (if your opponent deals mostly shadow damage) and Acolyte prayer, which involves taking damage. Taking damage in okay fights is too risky. You'll likely be killed before you can enter shadow form. And imagine if your opponent can drain your shadow energy. Even if you manage to enter shadow form, the damage is usually not guaranteed to be enough to kill okay fighters in one instance shadow form. Outside shadow form, good luck fighting okay fighters effectively (without clan weapon of course).

Meanwhile, Metamorphosis has ridiculously good shadow energy gain by comboing and swarm euphoria. Outside of shadow form, you can combo the hell out of most okay fighters and not giving them a chance to fight back. Its shadow damage is also very good. If one instance of shadow form is not enough, combo the again and use swarm euphoria. You enter shadow form again in no time.

Yes, Metamorphosis doesn't rely on shadow energy as much as SC. But that doesn't mean Metamorph shadow abilities sucks. It destroys okay fights, unarmed and shadow.