Where do I even begin? I take it you haven't played against a good Teemo or just haven't played against a lot of them in general.
_1. He counters pushing/snowballing/any kind of aggressive play. _
- Good shroom placement, hell even average shroom placement can effectively stop many champions from pressing any kind of advantage they have. Even one shroom can demolish a squishy's health bar. With Liandry's Torment + Void Staff, even tanks/bruisers have to be careful not to walk through too many. Playing against a good Teemo as a roaming champion is an extremely tilting experience, as he more or less passively removes you from the game.
_2. For the same reason as above, he can be extremely oppressive when he gets ahead. _
-When his team is winning, good shroom placement can keep the opposing team from going anywhere without consequence, all while his team gains vision of the opponent's side of the map.
3. His targeted blind is annoying and there isn't much you can really do about it except not engage.
_4. If he's played badly, he pretty much makes the game a 4v5/2v3, ruining the fun for both his teammates and opponents. _
Personally, I don't mind having Teemo on my team, as I'm not a good early game player. If he's a least average, that means our team will guaranteed at least make it to mid game. I hate playing against him though.