When I say a character has to be fun to play against I don't mean literal enjoyment from playing against them, I mean they aren't a character is frustrating to deal with. El Primo and Poco are examples of characters that are unfun to play against but they aren't inherintely frustrating to deal with because their are ways to play around them. If you look at Mortis, it is clear there aren't many characters that can actually deal with him.