Riot is unable to decide what role Kog'Maw fills

Alright, before I get my hands dirty I have to agree that pre 6.19 Kog'Maw was very powerful. OP? Perhaps if the enemy Zed couldn't assassinate him or wasn't one shot by Caitlyn's ridiculous trap/snare combo (which I think is absolute bullshit). Heres the thing though, in a meta full of ADCs that build like assassins (MF, Lucian, Jhin, Draven etc.) Kog was the only one that stood a chance at retaliating. These AADCs built nothing but armor pen and could kill Kog in 2 shots if kog was out of place or was simply bombarded with an MF ult. OP? Only if you let him. So they reverted him, damn.

Reverted Kog got attack speed passive on Q, the rest stayed the same. W's main power source was removed but range was increased early on. E and R remained the same. Since Kog doesn't build rage blade anymore, everyone decided to return to the old build. Triforce was different back then. Zeal was used instead of Stringer. Critical chance, MS all good on Kog because it would help him a bit more when building infinity edge. The problem with Stringer (on kog) is that he doesn't need CDR as an ADC. On top of that, if you have lots of CDR that means you'll be using lots of abilities. The problem is that Kog's abilities (though kind of strong) are extremely mana costly. Q costs 40 mana on an 8 second cooldown. W is 40 mana on a 9 second cooldown. E is 80 mana on a 12 second cooldown. R and repetitive uses of R are 50 mana while cooldown is simply the cast time (infinite as long as you have infinite mana). Now that the cost and cooldowns have been established, Kog Maw's mana pool is greater compared to most ADCs BUT with the cost of each spells in which you have to weave auto attacks in between to trade or in team fights is so little that its laughable.

Suppose you get ganked at level 6, you have sheen, and all your cooldowns are up. Kog'Maw has 730 mana at this point. In order to kite back/trade you "weave" your abilities and autos together to reflect damage. Q, AA, W, AA, E, AA, R, AA, R, AA. That's more than 1/3 of your mana gone. This doesn't even include whether you landed all your skillshots. Furthermore, the laughable portion of this is that all your abilities (except R) scale terribly with only AP. As an ADC you can't build mana/ap items so you're left with your ult to reliably trade. The ratios for max rank ult (doubled) are 300 + 130% bonus AD and 50% AP. Because you've built only AD items (Triforce, Runaans, BoRK, IE, LDR) your ult will scale well. Since Q,W,E don't scale with AD, we can assume you'll primarily use R to attempt to trade. Level 18 Kog'Maw with sheen has 1258 mana. In extended team fights you can only ult 6 (1100 mana) times to efficiently keep yourself from going OOM. You'll be doing mega damage with the sheen only if you're landing the very difficult skillshot. Even then its not a reliable way to trade with anyone.

Immobile. Current meta consist of fast paced games in which everyone builds mobility or cancerous anti-mobility to out play their opponent. Already immobile, this puts Kog'Maw and his team mates in a very bad spot. At 325 base MS he stands next to other ADC but what makes Kog stand out is that he is the worst ADC botlane because of his kit compared to others. For example, Ashe is another ADC that is very immobile but has abilities that help her survive (slows on autos and R), same with Varus, Jinx, and Cait. On the other hand, other ADCs have gap close such as Lucian and Corki that can be used either defensively or offensively. Kog'Maw has no self peel besides E which at max rank is a 52% slow every 12 seconds which people can easily side step or get out of. So if Kog'Maw gets jumped on and the support cannot protect him, he is 100% dead.

Honestly, I've had more success with AP Kog'Maw simply because I can cater to his mana costs and benefit from the AP scaling on all his abilities. Building tear then ludens echo gives me the move speed and enough AP to clear an entire wave with a couple autos on the melee and ulting the back line. This allows me to clear wave effectively and roam top and bot. Move speed, which is a problem for Kog since he's jumped on so often is a must on AP Kog to out run your opponents. MS quints, Ludens echo, Lich Bane, Rylias+Liandries are built on Kog because they synergize with him TOO well. AP Kog maw uses every ability (even W since it scales 1%=100AP which could work well with nashors/lichbane) to maximum potential to take out opponents without using too much mana.

I'm not sure what Riot plans to do with Kog'Maw but in all honesty they should just give up with the AD bull shit because in nearly every elo no on gives shit about protecting the X composition and prefer to play the game solo. I CAN play the game solo with AP Kog. Not having to rely on anyone is a blessing since I can just peel for myself while dishing out tons of damage. Frankly, they should just give up Kog as an ADC because items do not cater to him, MS is lacking, mana problems, and meta consist of cancer ADC that build youmoos/black cleaver which makes it a death wish to even walk into lane. But knowing Riot, for all I care make him a tank or support or something stupid.

or actually fix the fucking champion.