Fell
I was invited to a mostly pet undead group in GF, had a tank to pull agro off puller, but pets tanked... It was my least favorite group I've had yet, very inefficient too compared to a normal tank/spank setup.
No mana regen so being dependent on necro/BST mana to keep pets healed. (I am BST but if I sit to regen mana, I don't do DPS... Main reason I disliked the setup).
Having to let pets build agro because taking agro from them could be deadly without a healer. Mobs run in and fear or gate, lacking kickers meant when they triple cast gate, they end up gating.
I do think pets can tank well (if they can hold agro) they seem to excel at holding agro on mobs too high for the tank to reliably taunt... But require a lot of healing from owner and designated healer if fighting this deep of a red. Otherwise pet provoke is not the most reliable especially if your CC is an enchanter.