I mean, there's nothing stopping you from just getting killed as you're trying to get use out of your LB. You can just as well press this button and be immediately turned into an Imp and there goes your LB. Or, y'know, you just take enough damage to die, not like you can Recuperate during the casting.
Come to think of it, it isn't much of a Rush either, considering you can just... Sprint... Away from a GNB throwing that and he can't exactly follow, so it isn't even that damagey as well, and the Relentless Trigger kind of requires you to be in range of people that by then would've just run or know they can ignore it.
My mind brings me back to WAR's LB that allows him to have all Guard's effect (Minus damage reduction) AND sprint speed for a whole 15s... Seems to me that GNB should have the same during those 4s that he's trying to Rush.
Another option would be to actually just give him guard as he LBs, reduced movement speed included but damage blocking completely intact. So he can Guard into LB and making him the ultimate stall machine (But also allow all Guard-Breaks to work against him.)