It really varies on the person, but I would assume a scrub Akuma or Yang would be easier to win with against a scrub Yun just because the first two have a bit more lenient learning curve in comparison. With Yun, you have factors like Geneijin, how to land Geneijin, and what to do while NOT in Geneijin. Sounds simple, but it’s way harder because characters like Chun and Ken could easily put him down while he has no meter.
I don’t know what to say about mediocre Yun vs mediocre Chun, but I’d say just stick with Yun if that’s who you want to play. His combos are harder than some others, but you could potentially get 50% off of just one startup combo, and then like 5 seconds later be able to do another 50%(while managing to get meter back just from like two combos in between). A decent Yun has a good chance at stomping a decent Chun, imo
As for the tech roll, the timing is always the same: tap down the moment your character touches the ground. Some characters roll later than others, some knockdown moves and certain throws require different timings to be able to tech roll after.