On top of that, I'd recommend giving outposts some form of upkeep cost - placing them should require some thought, otherwise you can pump them out and drop them everywhere as fast as you can build them (which once you hit midgame, is often every single turn)

edit: Another possibility would be that all-encompassing borders simply absorb outposts, preventing you from using them to steal resources deep in enemy territory. Near borders, they should stay firm, but not in the middle of an enemy empire.

Do units get defensive bonuses stationed in an outpost? I didn't notice anything.