I'm obviously not max level yet but from what I've experienced leveling GNB and from what I've heard from others who already have it above level 70, GNB mitigation is somewhat lacking. Brutal Shell's shield is barely noticeable and the heal is pretty weak, their regen is also not very good. Their tankbuster cooldown is just a flat out worse Raw Intuition lasting around two more seconds but is 5% weaker (15% vs 20%). Their party-wide mitigation is magic only so useless against anything that isn't magic damage and since parry was nerfed (and was never good to begin with), camouflage is basically just a 10% damage reduction cooldown... Superbolide is almost as bad as Living Dead and can screw you out of benediction/essential dignity, only things it has in line with other tanks is Rampart and their 30% defensive cooldown.

They're a blast to play, really entertaining especially at level 70 when they do start feeling quite speedy, but they seem to be considered the tanks with the worst personal mitigation at the moment.