"Bolters" are small and weak but they're quick and harder to hit, where as "Bruisers" are big, chunky mother lovers that can give and take a lot of damage, if they manage to drag their sluggish legs quick enough to catch up to you that is.
"Retchers" make sure Overrun isn't just a game of chase; they've got the advantage of a projectile weapon in the form of toxic, luminous green vomit. They spit it at you. It's horrible.
Is Undead Overrun treading on the toes of Nazi Zombies? Certainly, but it's bringing enough to the table to differentiate itself and the third-person Red Dead world is the same rich environment we all loved the first time round.
The fun-receptors in your brain will be tickled in the same way they were when you played Nazi Zombies but at the same time there're subtle differences in the style of play and a massive difference in the arena.
Basically, if you like killing zombies over and over again, this is looking like one for you.