I have not really had too many bad experiences considering my playtime, but i remember two.
One was a loner/stealth zealot on Power Matrix that was just speed running without killing anything while activating every spawn on the map and left me and the other two players to deal with whatever enemies he agroed.
While i was annoyed by his lack of contribution to the run, i was kinda enjoying having to fight all this stuff and figured he would end up fighting when we got to the first event so i did not try to kick him. But when we finally caught up to him he decided to just stand on a box out of attack range while me an the other players were doing the event, so we ended up kicking him.
My second one was in a group with two less experienced players and a knife zealot, i felt the run was going well, we had good enough pace but i was also making sure the two newer players were keeping up while the zealot was kinda rushing it and dying when he got overwhelmed.
Eventually we finished the run and the zealot must have been annoyed because he was fuming in chat asking us why we were wasting time and going so slow, called us some less than savory things and told us to make better builds, which i found kinda ironic since he spent 1/3 of the run dead.
All in all, not to egregious considering i got like 1.6k hours in the game.