I think that there's way too much to do about 'skill expression' in the League Community. With the hatred for simpler champions and point-and-click abilities I constantly wonder why there aren't slews of threads on why AA's should be made into skillshots a-la Battlerite or Smite.
Do not 'decision making' champions have a place in the game? For their simplicity they usually have multiple tools taken away from them etc...
Hell, one of the hardest champions in this game is Singed. Despite his simple kit he requires a high amount of game knowledge and sense to properly pull off.
Simple champions also help with accessibility issues, reliability vs power levers during balance + champ select, etc...
As for Yi, Trynd, and Jax...
Yi - I think that his slow immunity, resets, and true damage should be removed from his kit and he abuses the new Rageblade way too hard, but skill expression?
A good Yi will wait out important spells and then enter a fight dodging key spells using his alphastrike.
Sources of damage can be mitigated using meditate, for how long he meditates is a risk/reward on losing his target versus dealing more damage.
Tryndamere - iirc during a podcast Marc Merrill himself stated that Trynd was an outdated and bad design even when on release.
Jax - Essence Flare is really broken, he shouldn't have aoe reduction on his counterstrike, and why can he autoattack during it?.
That being said, I think his main problem is how strong he is early on w/ how well he scales whilst still being a good duelist in lane.