While higher-profile marksmen such as Tristana, Twitch and Vayne have received more flak on the Boards recently, I personally think Kog'Maw represents some of the worst issues with the design of marksmen: as a champion with long-ranged, point-and-click damage, Kog'Maw's entire counterplay boils down to killing him before he kills you. However, in an environment where a) marksman damage is exceptionally high, and b) back lines are exceptionally well-defended, that counterplay is severely reduced. Moreover, the fact that his damage is target-agnostic means that the classes who have to travel the longest amount of time in order to reach him, i.e. fighters and tanks, are the ones that suffer even harder from his damage output. The fact that he is able to build a large amount of defenses on top of that, and receive even more protection from his support, adds insult to injury, as even at melee range it takes far too long to kill him relative to the time it takes for him to kill his opponents.
In general, the problem with a lot of marksmen is that their almost purely point-and-click damage means that they automatically put any target within range on a clock, where their victim is guaranteed to die within a certain time if the marksman is allowed to attack them unhindered. In the case of most marksmen, however, this is at least mitigated by the fact that their attack range is 550 units, allowing other classes to either outrange them or close a shorter gap in order to reach them. Kog'Maw does not have this limitation, however, and as such is capable of starting the clock much sooner, particularly since the steroid that enables this power has a significant uptime. He doesn't have mobility or hard CC, unlike most other marksmen, but neither of those weaknesses are as significant as they could be when he is constantly accompanied by a support, who can supply both lockdown and necessary protection, and can personally build a fair amount of defenses without cutting too hard into his own damage.
More generally, I think Kog'Maw is a good indicator that the clock every marksman applies on their target is currently far too short, as there are frequently cases of melee champions getting killed before they could truly get in range and deal significant damage to them, and even mages having a harder time dealing with marksmen whose mobility, range, or stealth allows them to kill them far too quickly. This is why I think Vayne and Twitch in particular have been unhealthy for quite some time, as their combination of upfront burst, stealth, and on-demand power (plus range, in Twitch's case) make them impossible to deal with properly past a certain number of items.