I love the one city challenge in Civ 4/5, so I figured I'd try playing only one planet. I suck, and I had no idea how hard it would be or not, so I set myself on a 16-star map against two 2s.
I won, but it was a grind. I didn't realize that by setting myself at the end of a corridor with only one wormhole, I'd be limiting my knowledge raiding options to that one adjacent system (since you need supply to raid, and I wasn't letting myself take any more systems.) So, that's the one of the two big challenges; you basically get a fixed knowledge cap of 13500 points, period... better spend it wisely. The other one is the total lack of an economy, but that's more an issue with patience, at least at that difficulty level.
I spent my ubervaluable knowledge points on... I think it was Mk2 Bombers, Mk2/3 Bomber Starship and a couple of odds and ends that fit in the rest. I neutered the first AI's homeworld completely except for his actual command center, and then made sure I destroyed both of them within a few minutes of each other... so the net effect is that my AIP was 4 to 6 for the whole game.
(Would have been negative, but that was the floor.) I don't know if it was the super easy level, or the total lack of AIP (or both, probably), but there was absolutely nothing so far as forces went. He never amassed an army larger than 60 ships anywhere. His waves were consistently something like 13 tech 1 fighters. He had a few counterattacks after failed attempts of mine to win which actually tested my turret system a bit, but that was the closest he came to scary.
Anyway, it's a different way to play. I recommend giving it a try once if you like that sort of thing. Once is probably all you'll need.