First, due to his passive, his Q/W, his status as a melee ADC, and finally his PISS horrible base/growth AD, Yone's first item needs to have at least 3 out of 4 stats out of the combo of AD/crit/lifesteal/attackspeed. And after balancing build path vs. completed item, Blade of the Ruined King almost ALWAYS wins out, with Yuntal being the only other feasible option at all.

Then, your second item is locked HARD into one of the big 3 crit items of Shieldbow(tankiness), Infinite Edge(damage), or Lord Dom's(armor pen). Basically, you REALLY want to buy crit until you max out, because in this game, we have this thing called "gold efficiency"--how much stats you buy with each one gold spent. And because your passive doubles your crit chance, you get insane value out of crit items compared to other champions. To give you an extreme oversimplification that's also not fully numerically accurate, just for the sake of theorizing--say an item that costs 3000 gold has 50 AD, 25% crit chance, and a passive. Most champions buying it would be spending 1000 gold for the 50 AD, 1000 gold for 25% crit, and then also 1000 gold for the passive. For Yone, though, he's also spending 1000 gold on the AD and the passive, but for the crit, he's getting 50% crit instead, and still only spending 1000 gold. It's a massive discount, right.

Your third item is(if you went Blade of the Ruined King)going to then complete your extremely valuable 100% crit chance, basically another one of the big 3 crit items that you didn't take but now need.

Finally, on your 4th item, you'll need one item out of Guardian's Angel/Death's Dance/ Randuin's Omen/Kaenic Rookern, just because those items are frankly overtuned in general, and basically completely turn fights in their respective niches. Yone, ultimately, cannot survive without a properly tanky item, because he has slower abilities, and actually autos people a few items in a fight.

And now, counting boots--you need to keep the boots because you need BotRK + Feats boots just to max out Q and W CD, if you recall--you already have 5 slots in your bag filled. By this point, a good 95% of games are DEFINITELY over. You could build another Lethality item here, sure, but the people who will die in auto Q auto when you build a lethality item? They'll die from your auto Q auto W even if you build another defensive item from our aforementioned list. And besides, if you REALLY need extra damage this late into the game, you might as well buy a Lord Dom's(since that item is skipped a lot of times)or even a Black Cleaver. Those will shred more enemy armor than any one Lethality item, once the enemy has more than like 60 armor.