Yeah, the jumping into a team of bots being crushed by a Player Team would svck. But yeah, I am not/have not been very clear on the modes myself. From my understanding the Campaign seemed to be the only mode
"- There is also a 'Stopwatch' mode, where you play the same objective based maps, but whatever time Team A beats a map in, Team B will have to beat that time." From H0RSE's compendium.
Stopwatch mode is like normal gameplay, except that you take turns doing the objectives. You play as both Resistance and Security and you have to complete more objectives or get a lower total time than your opponents. The only concern I have is what happens if you both fail at the same objective. Obviously, your time is used up, but I don't know if the time is set for the whole half of the game or if it is just set for that specific objective. I'm leaning towards the whole half, but that could make draws more common. We'll just have to see how it works.