I know it's silly, but I'd stat it up as no catch. The catch is that they don't have the toughness when not in their were-form. Take the power at full cost without a catch rebate and have a +0 unknown catch.
The whole point of a catch is to give a workaround or weakness. The weakness of an Alpha-style shapeshifter is that they're squishier when human.
I know RAW says Toughness powers must apply to a catch, but then you've got Werewolves without one all over the place. Alpha style werewolves don't have a catch like Silver, so why would other were-creatures?
In my games, as long as you don't have access to a toughness power all the time (either against a certain type of weapon, or when shapeshifted, or only during a full moon, only carrying a certain item, or whatever) that's good enough for a catch.
When we had a werebear fighting in our party, I did throw them up against Circe. She made a declaration that her magic was focused on shapeshifting, so it bypassed the toughness powers of shapeshifters. The player got a fate point and was scared for the first time in the entire game. It was fun.