Some of their buildings give negative PO, but honestly, it's just free exp and money. As long as you have a general relatively close by, enjoy the free resources.

You should have 1 of the big boss tents(whatever chain unlocks the goblin big boss) in every city of a province and later also get the shamans hut. That alone should be sufficient to maintain public order after a few rebellions and you'll never have to worry about it again.

Those 2 building chains do way more than just maintain public order though as they're very useful to your city garrisons and pretty much mandatory for a city if you're playing head to head.

EDIT: Also if for whatever reason that still don't cut it just build the max tier walls chain in your cities, you can auto resolve any rebel army with those and green reinforcement is redonkulously good. There's also 2 obedience researches on the tree.

Post battle and raze options provide factionwide public order.