He was a hypercarry but more versatile and he was a really fun midlaner too because his ult. was great for poke. Unfortunately, they're not bringing back the old ult. AP Kog will probably be shit now... it's not even a revert... it's actually worse. W used to be a flat 6% max HP and cost no mana.
As far as ADC, he didn't have the "i just aced your entire team in 5 seconds" thing like after the rework but his single target and tank busting was better late game.
If I had to guess, I'd expect Kog's 6.19 winrate to go down to 46% ADC and 44% mid lane.