I don't think it's viable at all. This is coming from an ex-Kog'Maw adc main.
His W has such a long cooldown and his wave clear is awful. Only way to carry with Kog'Maw is to get super fed early or get to late game.
AP Kog'Maw, on the other hand, has extremely strong burst damage with his Q, E, Ulti and being capable of proccing ludens with his AoE skills.
AP Kog'Maw > AD Kog'Maw.
InSingedWeTrust2 votesAD Kog'Maw has the most damage potential in the game.
InSingedWeTrust1 votesAP Kog > AD Kog
Tell that to the popular Juggermaw comps built AROUND a Kog'Maw ADC in LCS.
arcinex1 votesAD Kog'Maw has the most damage potential in the game.
I just played AD Kog'Maw but guess what? AD Kog'Maw is basically a hybrid AP.
http://matchhistory.na.leagueoflegends.com/en/#match-details/NA1/1839601772/226866899?tab=overview
Check statistics tab for PD/MD spread.
Lugg1 votesAD Kog is completely viable, but I think AP is stronger.