I don't really want to get into it, but no matter how much prior investigating you do will not reveal, among others things, (i) a closet homosexual or (ii) simply the fact that they do not "get along."
You could have either, but with respect to the second, no matter how "temimusdik" or "chashuve" or upstanding, genuine, honest (and append any other positive attribute here) the two are, their personalities may simply not "jive."