Hadn't noticed the bias toward SI in versus Hard Rain, both teams just require good communication (microphones, with the ability to tell where they are, especially in Realism). It's a good map, with great opportunities for the Survivors to hole up while there's a horde, and equally so for Infected players with corridors, places of no return and hiding places for Spitters and Smokers. I'd say it's a great map for versus too.

The Passing can be trickier with realism, but that might have to do with the relative newness of the map. Swamp Fever is something different however.