AI, uneven teams, DG just plain being on the wrong side(UB on forces of light for example).
End of JagerJack's quote
You're right, there shouldn't be AI in the games, that is silly. But in my experience, it rarely happens. Certainly nowhere near "half" of my games.
As for DGs being on the wrong side, I have not seen this once in my admittedly limited sample, but I feel safe assuming it is quite rare.
As for uneven teams, how is this more likely in pantheon than in customs? Assuming you mean skill-wise. Unless you mean the (again very rare) 3v2 or 2v1 situations -- which I haven't actually had occur in the last 2 patches, but perhaps I'm just lucky.
I do not think it's perfect, but again, I do find it quite functional and a fun way to ensure a PUG v PUG. I'm sure some people are trying to abuse the auto-matching to get their premade in, as you stated. But surely this is rare. Why wouldn't these people just play custom? After all, they all go towards your statistics/rank on the website, and it doesn't involve you having to go through a bunch of hoops just to get your team into the game.