Not programmed and designed properly. Too easy on easy and too hard on hard(solo on), crawler zombies has too much health on push though they look the same as common zombie and not every zombie should be able to sprint, since the tanks (the black and tall zombie) has proximity damaging and very tough to kill even with explosives or with headshots, also their chance of spawning is high, so there usually would be 4 or 3 of them in one wave. This game is really broken, should've in early access