Even if they weren't so infectious, only the more dangerous Apocalypses could hope to outmatch them even with a gargantuan numerical advantage. The basic, entry level Slasher Necromorph is utterly immune to small-arms and almost immune to most forms of conventional attack. Anything without at the very least superhuman (two or three times human) strength or highly advanced or specialised weaponry isn't going to be able to bring them down permanently at all. They are also strong enough to rip apart steel plating (albeit slowly) and can decapitate an unarmoured human with a single swing of their scythe-like claws. The most dangerous ones, like Hunters? Wolverine-level regeneration and the strength to casually rip a human being in half. Anything short of total disintegration simply will not do the job. One Hunter or Ubermorph could likely solo a Romero-style Apocalypse.