Glass Yi is bad IMO because you get killed in 2 crits by an ADC if you get stunned.
Bruiser Yi with a randuins is the scariest thing in the game because if he gets to carries, he still kills them in 2 seconds, but if he gets stunned, he also tanks substantial damage for his team with armor/HP from items, multiplicatively scaling with W. If he gets peeled, NP, he can stick to targets with 3 slows (blue smite, randuins, BoRK) and a resetting gapcloser.
You said later that he isn't as good as Warwick. But that's because of your build, not because of Yi. With randuins, Yi is stickier than WW because WW has no gapcloser outside his ult. Once he burns his ult and the target isn't dead (due to peel or just tankiness) he gets kited like crazy. Warwick is inferior to Yi in all aspects except he has the ult. He farms way slower, doesn't do anywhere near as much damage and all of his tankiness is from items, just like Yi. If Yi builds tank, he does way more damage than WW and is just as tanky, and most importantly, he can actually buy those items through farming, without having to take kills from every lane.
I am a 300+ game over 2 seasons Yi main in plat 3.