one of the biggest things that holds kog'maw back is life steal
he can't lifesteal through his magic damage. typically ADCs build AD life steal items like BoRK, BT, DD, MCS. However, those ADCs benefit from those items immensely compared to Kog'Maw because either their abilities scale with AD or have some sort of amplification (crit items) that greatly boost their life steal. Kog'Maw on the other hand builds BoRK, which is decent on him, and life steals 12% of physical damage dealt.
The shocking thing is that Kog'Maw does not heal from on-hit effects of BoRK or Runaan's side bolts. So the only thing that is keeping him alive are his auto attacks. Kog'Maw builds very little AD. Assume an endgame build nets him Rageblade, Runaans, BoRK, Bers. Greaves, GA, and Wits End. Level 18 total AD = 98.4 + 35 + 40 + 40 = 213. With 12% LS (without crits) you get about 26health and with a crit it'll net him 52health. Those numbers are a complete joke especially since that is all you'll be relying on during a team fight in which you'll most likely be blown up instantaneously.
It's actually pathetic that he is gated this way. Picking Kog'Maw forces your team to pick a support with heals/shields, CC for peel, forces them to purchase Ardent Censor for heal amplification, and create a team to constantly protect him. However over the years its been increasingly more difficult to create the "protect the kog'maw comp" when you can pick champions like Twitch or Caitlyn who have the ability to heal tremendously, abilities to help them escape, and a safe laning phase.
I understand that Kog'Maw on paper is a late game monster and there needs to be things to prevent him from becoming "disgusting" but he only heals from his AA and its only a measly 12%. This is a huge barrier to keep him alive and should be justification to opening up his kit a little bit and making W consume no mana to use.
thanks



