Some things never change... (and some need to)

Every time I see Berserker teams, I'm still like "damn Berserker OLAF is busted."

It literally is Olaf. And only Olaf.

The other 5 are just to activate his 100% splash damage and extra damage. Like, Renekton is good for the eggroll, and Voli is nice for Electic comps...but those are supplementing people for their other traits. The ONLY reason you go 6 Zerker is to hyper-stack your Olaf. there's no strategy to it. It's "oh, I have good Olaf items, and I found an early Olaf!" throws in 6 zerkers with perfect olaf items, and just walks away from their computer on hitting lv 6.

That right there, instant top 4. Upgrades? Other than finding Lv 2 or (god help us all) Lv 3 Olaf, who needs them? They're nice to have, but they're not even fucking necessary. You automatically acquire Glacial from running 6 zerkers, so that "20%" freeze chance starts freezing 3 people at once on every 2nd autoattack of the round, and by the time anyone close to him can move again, he's ulted and charged up his rageblade, so there's your permanent freeze that NOTHING can stop, cause Olaf can't be cc'd.

Fucking change his ability to Undertow, you already gave Reksai a True damage ability, there's no reason you can't give it to a Berserker. It's still loads more balanced than "I instantly win the entire game from freezing everything near me off of my 2nd autoattack, becoming unstoppable, and 1v9ing your fucking team."

THEN you can balance Zerkers as a class, and not a singular unit. >.>