I don't think he's a grub.
For me a grub is a fairly average player who compensates for a lack of ability and builds a career mostly around dirty tricks and deliberate foul play. Darren Senter, MIck Ennis, Glen Morrison, all grubs, (but each played a fair amount of footy in their career.)
Luai is more in the Billy Slater/Terry Lamb category of a great rep player with a nasty side. Dirty? sometimes, but can also win you a grand final or an origin series. I'd rather have a great, ultra-competitive half in our side, than say, a polite and quiet bbq enthusiast.