Consequences are chosen by the player of the one taking them, right? So if you have Mythic recovery and your arm gets chopped off as a consequence then that could just be a severe consequence.
If you have Mythic Recovery (and regrowing limbs was appropriate to your High Concept that gave you access to the power), then it's not unreasonable to mark off the severe consequence box, write down the consequence of "Dude, have you seen my arm anywhere?" and have the arm grow back as you recover from the consequence a few minutes later. But if you mark off the extreme consequence box, then the arm isn't going to be growing back soon, regardless of your Mythic Recovery. Perhaps for such a character, and extreme consequence needs to be worded differently, since limb loss is not 'extreme' to him. Perhaps in his case, his new aspect should be "But ... its
always have grown back before!" which would reflect not only the loss of the arm, but also a loss of confidence is his powers, which until that moment have always made him all but invincible. And until he regains that confidence (and finds a way to regrow the arm by another means) the extreme consequence stays in place.