Zombies (the animated dead) only work in magical settings.i have actually got a pretty detailed zombie plan that will work against a traditional "walking dead" style shambling hoarde, although if we're talking world war z style piles of bodies it gets a bit trickier.
So it's not the zombies you have to worry about, it's the necromancers. Well, and the zombies.
"The Walking Dead" and suchlike just aren't possible. Movement requires energy, and the dead have no means to turn food into energy. Sure, sure, they can "eat" brains, but they aren't receiving any nutrition or turning their food into any kind of usable energy. Mostly since their circulatory system no longer functions, neither does their digestive process, etc.
Even in D&D zombies are rule-breakers. Whilst physical dragons can fly due to the laws of physics, dracoliches whose flesh has entirely decomposed can still fly, by "supernatural" means. It's BS. Won't somebody please think about the practicalities of being undead and apply some logical consistency? It's serious business.
There was a new zombie arrival in a certain popular TV show recently, and I was a bit upset with how they handled that too. Zombies are great and all, but to make them work you really need to think about how they work, and not just do things because they're "cool" or necessary for the plot.