GNB in DPS stance is only a little less tanky than MNK. MNK also lacks any sort of mitigation and is pretty much entirely reliant on burning dash charges for a personal shield or a really shitty personal Macrocosmos that doesn't even pop automatically. GNB lacks a mitigation button or the shields, but No Mercy basically means permanent 20% healing up as long as you're fighting, which is... actually pretty strong. Especially since MNK leaves themselves open to being kited if they spam out their charges to keep refreshing the shield (and a lot of MNK's utility is also in giving those shields to others.) MNK is tankier, but not *that* much moreso.

Maybe they could give GNB a nerfed Nebula available at all times and tank stance buffs it? But they'd absolutely have to walk back a lot of the potency and No Mercy buffs since then if they did that.