I've had no problems, but I do it only for day runs class IV- or easier. It's fun meeting people who you wouldn't otherwise cross paths with.
For class IV+ runs or above, you want to be careful because you rely on your partner for safety.
You can also just go to the takeout or putin for a popular run and ask join up with others. That gives you a chance to give them a once-over before joining them.
It seems most kayakers are not total screwballs. How can you mess up a good day on the river?
The worst thing that can happen is you end up rescuing someone repeatedly. But, that's comes with the risk of joining others. If you agree to have them in your group, you've got to look out for them even if they don't belong there. I see it is a contract, like it or not, you signed on once you agree to boat with them.