Basically it feels to us like we missed some restriction on using robots because as they are, they single handedly made several building obsolete before they were even added to the game.
Minions can only be placed on buildings in your own Charter, and only on unoccupied buildings. They're also never bumped, so you can only use each one once before recalling workers.
Also you have capacity for minions. Each minion carried between games must be of a different type, even if you have the capacity for more than one.
Those are, I believe, all of the restrictions on using robots.