[Bullock]

I also gave it a try in a multi player, with 7 of those.

It has a poor summon, whatever the type of prov or power of dom.
i think one spawn every 2 or 3 turns.

they can also be useful to defend from indis and maybe even take back lands in later stage of the game if you keep them roaming in your lands with their summons.
I don't think you can get enough spawns to plan any serious strat with them.

Globally i don't think it's worth the 6gems.