But why? If we can lengthen two segments of a bond structure at once then why cant forearms be lengthened? + for the "hand" ability don't just make expeculations, prove that it can actually happen.
It could be that it varies from tibia/fibula lengthening because there is rotational movement involved with your forearms. This movement I'm sure makes it more vulnerable, and will increase chances for something to go wrong.
You can find a diary on the other site where a girl had forearm lengthening. I don't remember if it was a cosmetic case or correction but it didn't turn out very good.