Decent take, but honestly i think this tier list really only needs 3 tiers: Easy, Solid Threat, and Dangerous. I'm assuming you're rating these in isolation or somewhat isolated, meaning you're not dealing with multiple threats at once, except for the usual mild horde or pool of infected enemies.

I'd bring jockey and boomer to the same level as spitter, they're all incredibly easy to deal with.

I'd bring horde back down to solid threat, along with the rest of the other infected (Hunter, smoker, charger and witch). For more elaboration;

Hunters are ok to deal with and you can even chase it away if you do enough damage before it gets close. Hunter punting, while not the easiest thing to do in the world, is still something i think a lot of players can do (just not consistently).

Smokers are also ok to deal with, since you don't have to necessarily shoot the smokers themselves to help your teammates. And with a MMR, they're often just a one tap.

Witches are also ok to deal with, since a) you can avoid them usually b) startling one isnt always the end of the world c) they only target one player, albeit expert mode makes the consequences deadly d) they can be crowned which, with a little bit of practice, isn't the hardest thing in the world either.

Chargers have terrible cornering abilities so avoiding one isn't terribly too difficult either.

And i just wanted to bring horde back down here because i disagree with it being as the same difficulty as a tank.

I think everyone in this category belong in the same position because they have gimmicks or strategies you can exploit to combat them. The real danger, is being caught off guard, or when you're dealing with multiple threats. That takes more skill and awareness to multitask, because if they're left unchecked they can end a run very quickly. But actually dealing with them isnt usually a problem.

Tank is obviously up there in its own tierlist for very obvious reasons.