Adding flaws to "Alternate Form" ?
 

So from my understanding, Alternate Form is the Multiple Effects power with the Activation flaw.

Is there a reason there's no option to add other flaws to either Multiple Effects / Alternate Form, such as uncontrollable or unreliable? From my understanding it's legal in the rules, perhaps I'm missing something?

Ah, that was possible in 2E, when Alternate Forms were built as 5 PP Containers of 5 PP that they could have Extras and Flaws. 3E did not carry that over, although Quirks like Activation serve a similar purpose, the value being less than a full Flaw because once you have an alternate form up, it pretty much stays up.

The other option, of course, is to go with Complications. That works especially well for an Uncontrollable form like The Hulk or Lycanthropy where you may be able to induce it on your own, but it's just as likely to happen from outside factors, and you may not be able to exit the form at will.