Yi's has a few weaknesses. True, saying "just cc him" is very broad and frustrating to hear, but it can be better constructed as "his ult makes his immune to slows, but not stuns. If you stun him and use that time to run away, his ult will run out." His heal can be interrupted by knockups, or if you're an aa dueling champion like
you can autoattack him during it and he can't stop you. His alpha strike is annoying if he's good and uses it to dodge cc, but if he uses it for damage you can retaliate with cc or a powerful spell. Outside of his spells, Yi is auto reliant and while he can deal true damage, there's always
to piss him off.
Does that help?
One thing people aren't told straight is that all-in supports like Thresh or Leona (as in they stun/hook you and then you die) is that they need to be poked down. The key is to either hide behind minions or bait that hook out. If they miss, or you're safely behind minions but are in aa range, you need to poke them down. Leona is a tank, and plenty of Thresh's will build so, so poking them down takes awhile. But the cooldown of their main moneymaker will give you time to do that. If a Thresh misses his hook and it's pre-6, you also have a chance to try and nab the adc since he doesn't have an ult. Leona is more difficult, because of her Q, so it's a better idea to just poke her down and wait for her to leave lane.
People can't be super in-depth during game, but there's a few tips to help people out. They're super annoying, but once you get some practice in they become manageable.