The immunity power on the Wiki is way overpriced. The standard immunity assumes that an effective +8 to defense rolls makes you immune against someone with equal skill because they are never going to hit you. Then, with defense bonuses costing 1 refresh per overall +1 bonus, assigns Physical Immunity a base cost of 8 refresh. I'd call that a fair pricing, given the catch and it being limited to only stress and a single type of stress at that.
I'm pretty sure that's not the logic behind the pricing, given that
a) Physical Immunity makes no reference to defence rolls or skills
b) you can still be hit with +8 to defence rolls after a bit of maneuver-stacking
c) there's nothing in the rules letting you trade Refresh for defence bonuses 1/1.
Anyway, I think the cost of the Power is pretty clearly unfair when you compare it to other defensive stuff. Mythic Toughness plus Mythic Recovery is so much worse than it, but it costs much more.
Compare with someone having Mythic Speed, a "Monk Training" stunt/power giving +2 to athletics defense when unencumbered, and an "Elusive Target" stunt/power allowing them to always use Athletics as their defense skill. That's 8 refresh for +5 to defense, +8 when sprinting. Your chance to hit them with a normal attack is what, one in eighty?
But that guy had to spend an apex skill slot. And he has a semi-significant weakness in that he has to be unencumbered to use his full abilities. Opponents can bring him down by stacking Aspects or by grappling. (He'd have a decent chance of escaping a grapple with an effective 8 Athletics, but there are some really good grapplers out there...)
Whereas against Physical Immunity guy, most opponents just can't go anything except slow them down.
Also, that interpretation of Speed's dodge bonus is debatable. It gives +2/tier to Athletics while sprinting. Does that apply to dodge rolls made the same turn as a sprint roll, or just to sprint rolls? Hard to say.
PS: Assuming a Fantastic attack skill against our dodge-master, the opponent has a roughly 6.5% chance of hitting with no invocations. At least, that's what the internet tells me.