SMAC is the acronym for "Sid Meyer's Alpha Centauri", civ like on an alien planet.
One thing I really didn't like about SMAC, is I'd own a city of mine, perfectly happy - with a large army inside it... Then all of a sudden, I get "DRONE RIOTS - YOU CITIZENS HAVE DECIDED TO JOIN THE XXXX" and I lose not just the city, but all my military units inside. So now XXXX has become really strong.
I chalked it up to a bug and thanked the autosave feature. Rebellions are useful IMHO, but sometimes weird rebellions like that get annoying.
I never had a city leave straight away. I can't remember losing a city, but usually they'd have to be rioting for a few turns before that happened.
On the topic - the reason why rebellion is unpopular is because it's a negative punishment. Getting 20% less cash each turn is a different punishment to losing a city. With the former you are still growing, but just not as quickly. With the latter it is actually a step backward. We generally don't enjoy negative punishments that aren't dished out by enemies. We don't enjoy the latter either, but it's more bearable.