I haven't dated in almost thirty years but I think what would be the worst is finding out someone is heavily conservative when you aren't. Or discovering six months into a nice relationship that he's a closet antivaxxer or something, basically where you find out something that really goes against your values. Thankfully many of those people are obnoxious and loud.
OkCupid used to map those important things people don't talk about via thousands of multiple choice questions, and you used to be able to build a search filter from the answers you'd accept. Then MatchGroup/capitalism/puritans wrecked it. I don't know if there exists a good dating site anymore.