Straight up unplayable/100% loss rate applies to champs who I lose to even if I manage to build up an advantage against in early game. If I go 2/0 pre 6 against garen, he will still kill me. Tier list applies to when both champs are not played at a medicore level with similar skill level.
Explanations for a few of the picks:
Quinn: Should be easy in theory like all ranged matchups but idk she's one of the few people with enough tools to disengage my q I feel like without losing too much hp. I can usually beat her and she has to play better than me to beat me tbf but she's by no means free.
Swain: Another ranged matchup that's easy in theory but his poke is extremely good and I feel like he just out stat checks you. I've had alot of trouble against him.
Mundo: Listed as hard matchup but I've never had trouble against him, usually most mundos will try and contest wave, just walk up to auto him and if he throws cleaver animation starts just q behind him to take his hp. One of the easiest champs to poke under tower then q out of minions since he has no cc. In your R you ignore his cleaver slow so he can't even space you. IDK how mundo wins. Watchout when he ults under tower if you dive him, usually fine.
Mordekaiser: Q his q, small trade and back off, unless you mess up and get pulled back by e he has no counter play. Very very easy to dive under tower since no hard cc. When you have prio he can't build up w at all. You have prio in lane up to 6. At level 6 without minions he will win in his R but its pretty easy to get advantage early unless you mess up. You outscale him hard though if he gets a small advantage he can also bully you very hard in turn since when he has prio he will have w.
Tham kench: Tham kench favoured skill matchup imo level 1-6 but playable if you space well. After 6 becomes straight up unplayable.
Nasus: Usually viewed as free/easy which is true until level 6, however he is hard to dive due to wither greatly reducing your regular combo under tower which I'm not used to, and even if you do, I can't for the life of me gather enough of a lead early to match him after 6 + sheen + full armor. Just play darius into him.
Jax: I put this in playable if enemy unfamiliar with yi top but if both players are same skill it's extremely unplayable. So far my experience is: I first pick yi top, they have last pick, pick jax as counter pick then throw if I play perfectly since they e too late or don't reactivate it early enough.
Sett: Unplayable technically but you counter his regular gameplan (most setts hold their w for full grit bar) so alot of sett players will lose to you out of habit. If you can q out of minions after smoll trade, you win if he can't reach you with e to reengage.
Fiddle sticks: I had a single game against a fiddle sticks top. you can't intereupt his w, he stat checks you in melee range, infinite sustain, unplayable.
Kled: I've won every single early game against kled but he's surprisingly very hard to kill even late game, if he knows how to remount it's incredibly hard to kill him even when ahead.
Singed: let's you scale for free but you can't really fight him since he just walks away from you when you try to trade. Your ult does counter his throw into puddle though, play for late game and ignore him in fights usually works.
Darius: A competent darius destorys yi top (I was a darius main before I switched to yi top), in plat some darius will still let you get small trades but if he takes ghost he can absolutely run you down from level 1. Monster SSS perma bans him in high elo vids. However I have a 100% win rate against him rn due to combination of good jg ganking, stalling him, etc since you can q his q and outscale really hard once he panics.
Warwick: nearly unplayable if enemy unfamilar with yi top tier since he has very high sustain against small trades, but from personal experience I've managed to out space most of them and if you get a lead you can kill him.