no it's not. at all. his ult already makes you ignore 50% armor.

For 15 seconds too. Which is plenty of time to kill everyone.

Because being able to shred tanks without needing your ult is incredibly useful, and it's great as a third item.

If the game isn't over by the time you get it, you need it, which is the logic for buying it.

The cooldown of his ult at max rank is 30 seconds, 15 seconds of that you have your 50% armor pen. So at most, you have to wait 15 seconds to get 50% armor pen.
The gold could be spent much better elsewhere, such as a bork, or even a maw if its an ap heavy team. I would ONLY consider a LW if there are more then 2 tanks on the enemy team.

The issue is it's dependent on you getting a knock up. If you have the tank in your face right now, you won't necessarily have the time to get the 3 q's for a knockup. And relying on your team for a knockup is not exactly a game plan.

Actually, it ignores 50% BONUS ARMOR as said in a Riot post. So with a base armor of 80, that's a 44% damage reduction plus the other half of the bonus armor they got and you have over 50% which is a lot.
I also thought about getting Youmouu's Ghostblade or Black Cleaver on him before and I've bought Last Whisperer as a last item because most of the time, people don't buy armor in my MMR for reasons I don't understand.
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Btw, I upgraded trinket literally as soon as I backed when I hit 9. Proceeded to ward everywhere. Even bought pinks and stuff. Still couldn't carry.
Feeding duo adc bot is really terrifying.

I've been kinda trying gnar top with smite for a few games, took cinder hulk and early level 2. feels good for now. I'm pretty sure i need better runes and masteries for gnar though pffffftttttttttttttt

Cinderhulk is dying. I only really see it as a nice option for tanks that deal no damage.

what build should you suggest

if u can't get ur ult off, no matter if it's by your team or yourself... why are u playing yasuo
he pretty much does nothing without it. that's how he's balanced