Late game champs that don't require good teamwork are unfun for 9 out of 10 players in the match.

Your team as a whole suffers early game when you have a late game champion on your team, but when they don't require strong team play like a Kog'maw does, the other 4 players on the team feel useless. You're putting all your hopes on one player, without feeling like you have any good way to help them.

Yi is the most extreme example of this - you'll suffer hard as a team early game from the lack of pressure most jungler bring, and the rest of his team will probably never be able to recover from this, even though HE becomes god once he has sated. You don't feel like you can help him much, except for perhaps "baiting" the enemy team into blowing you up with there ults, which certainly isn't satisfying.

When you have a Yi, you are betting the game that the enemy team can't coordinate and/or doesn't have the point and click cc with no travel time needed to deal with him - everyone else on his team feels more or less irrelevant.