In my experience in the few pugs I have played... You don't need any of the higher level stuff (wallpogo, syncs, etc). Pretty much all you should know is regular rocket jump, walljumping, airstrafing and at times speedpogo to play an effective roamer (The rest comes down to game sense, aim, communication, etc) .
Very rarely are you going to use rampslides, syncs, water, etc though they are good to know (e.g. fancy jump from mid-spawn to end-spire on badlands or water jumps on cp_well and that other map with the train)
Demo uses rampslides in their rollouts a lot (e.g. badlands, process, snakewater), so if you plan to play him, learn how to do that effectively.
When you jump as a roamer (or even pocket), you have to be comfortable with your jumps, be consistent and don't mess up all the time. The rollout (on 5cp & koth maps) you have to do usually only consists of regular rocket jumps, and even then you usually have to stay with your medic anyway so you don't have to do it extremely fast.
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