Under that argument the Rat King is a "fantasy monster" as much as a bloater is, which was featured since the first game. I just see it as another variation of the infected, a very exceptional and rare one but plausible, and freaking awesome to look at. It's not really a "big departure", it's just a group of clickers blended together like rats because of very peculiar environmental circumstances.
Scars rely on hiding and shooting from strategic places, having a full knowledge of the terrain, ambushing, psychological intimidation, and picking apart the WLF in small groups. Their approach is similar to guerrilla warfare. They just don't go around shooting arrows in an open field against gun-armed men like retards, unless they're left with no other choice.
One of the major themes of the game is tribalism and extremism, specially because of cultural/religious beliefs and territorial control. The whole freaking game is pretty much a reference to the never ending Israeli-Palestinian conflict. That's why the seraphites are so peculiar, to accentuate the contrast with the opposite faction.