Yeah but if he's hit while activating it he still pulls it off. :/
Yeah, its called "Unstoppable Ally" and it's just that: unstoppable. If Sly's on the ground and he hits that super button, nothin short of the apocalypse will stop Murray from bone-rushing you.
It doesn't need to be unstoppable though. I see no reason it needs to be the only lvl 1 that unstoppable