That’s a very good question. The other side of it is that there are people with psychological issues who do have good marriages. I can think of at least one person I know like that – someone whom I thought could not possibly get married and would never have considered setting up.
It seems to me that it might be a bigger problem if you are setting this person up with a friend of yours as opposed to if you are an official shadchan setting someone up. When you set up a friend, they are trusting you that you are only setting them up with someone who doesn’t have issues, a quality person whom they would set their own child up with (or go out with themselves). When a shadchan sets you up, you don’t make those assumptions.