There are plenty of champs who are mostly "useless" early until a certain level or a certain number of items, so in that part master yi is no exception. The exception is the "starts to 1v5" part. Few champs can say this. Maybe a yasuo or vayne besides of course yi. A few more can come close depending on people's picks... like jax or trynd.
So I've said this a few times now and I'll sum it up in short again... Riot does a poor job of balancing certain champs because they don't understand and respect power curves. Power curves is one of the two biggest ways to balance a champion, and rather than set it so that every champ's net power curve is nearly equal... they overbuff some and leave others under-tuned.
If you want detailed explanation/example(s):
What do I mean? Let's look at a few examples to illustrate the idea. Lucian is a strong laning adc. But his range and the fact he doesn't build quite like most crit adc's means his damage falls off a bit late. So early and midgame we might rate him at an 8 and a 7 out of 10 on the power curve scale. Late game due to the challenges he faces (his damage is still very decent if he's allowed to deal it) he gets rated around a 5 out of 10. So if we average this... he gets a rating of 20/3= 6.7. ... If we next look at kog'maw we give him maybe a 3 out of 10 early, 6 out of 10 mid and 9 out of 10 late. *forgive me if you think this is off; I don't main adc, but am guestimating for illustrative purposes. Anyways, back to the illustration. 3+6+9=18 18/3=6. So comparing kog and lucian... their power curve ratings are fairly close. This means that they have a fair power curve in comparison to each other. If some other adc were to come in with a rating around 8 or around 5 then they'd probably need to get looked at and buffed or nerfed accordingly.
But that's only half the battle with power curves. The other half was illustrated nicely with crit champs being unviable not too long ago. If we compared their power curves to those of the mages that had supplanted them in bot lane, their overall power curves were probably comparable. So what was the problem? The problem is when Riot goes to extremes with a power curve. For instance, we might give full build crit jhin a rating of 9 on the power curve. But in order to balance out that extreme late game power, he must be given a rating of something like 2 in the early game and maybe 5 in the midgame. Well if you have something with a rating of 2 vs something with a rating of 7 or better in the early game... good luck reaching your late game power spike. Simply put, the ends of the curve (early game and late game) are too extreme. And thus while crit adc's were "balanced", they were not balanced in a way that makes them fair for either party. This is why when crit adc's got a buff to their early game via item changes and stuff, they got some small reduction to the scaling of those items; you can't take something that's supposed to be weak early and hyperscale late and buff it's early game to make it average without shaving off some of it's scaling. Otherwise there is legit no reason to take some early or midgame focused champ... hyperscaling simply becomes the only valid option unless you know you can end the match in 20 min or so.
Yi, fiora, jax, and possibly trynd fall into a similar situation. Their late games are incredible, so their early games should leave strong early game champs EQUALLY incredibly stronger than them to give them a fair power curve compared to these champs since the early game champs fall off about as hard as these champs scale. But if Riot were to do that, then you'd end up with these champs being unplayable due to how hard they would be bullied early when in matches with highly skilled and/or coordinated players. And so... much like crit adc's they get some extra help early compared to what they maybe should have gotten. Only unlike with adc's (please don't crucify me yet adc mains), these champs don't/didn't get the same type or extend of reduction to their scaling that crit adc's did recently when adc's were given a buff to their early game.
And so you have this very awkward flux between stuff like fiora, jax, yi, and others being weak if their early game is left properly punishable to the extent they should be getting punished early game by skilled opponents on lane bullies/dueling early game junglers and being op when their early games are made competitive enough that they can challenge early game champs on near-even footing during the early game due to how little risk there is for them early - essentially their power curve is simply made much better (like average of 7 vs average of 4.5). Small changes to meta, to items, etc. can flux that pretty meaningfully for champs like yi. And thus, due to hit kit and scaling, he's just not set up to be properly balanced without regular attention. He needs a new kit - along with many other champs - which allows for more skill expression and more meaningful counter-play. A kit that gives him more competitive scaling throughout the game rather than leaving him at extremes at various points of the game - sometimes being very weak, other times being stupidly strong. It's fine for a champ to be stronger than most others at a point in the game - that's why lane bullies and hyperscaling champs exist. But when the difference in power at any point in the game becomes so much (strong or weak) that the champ is unplayable (due to far too weak at that point in game) or makes other champs unplayable (due to far too strong), then you have an issue. What I'm trying to say is that you can't like... make a champ have a power curve rating of 1 at one point ant 10 at another - it's not fair or fun for whoever plays that champ when they're at a 1 and not fair for their opponents when they reach a power of 10. I think a +/-3 is about the biggest acceptable gap without having potential for massive abuse at any given point in the game.
Anyone else, he outruns, and easily has time to do it if he heals your teammate(S) (Olaf also is purely countered by Thornmail, where Yi is NOT)
, which are both antithesis to tank items. Thornmail may affect Yi, but not before he kills the people he's attacking.
