The hordes would be absolutely wonderful, an explosion leading away a large part of the zombies, making a huge bulk of them as they themselves make a lot of noise, attracting single roaming zombies.
To deal with them you’ll either need to trap them in a place with natural walls (since z-levels are upcoming anyway) or build some way to stop them, for instance stone walls as long as no hulks or something similarly strong is there.
It should be a fairly uncommon event thought, maybe the first checks starting at year 2 or 3, but the amount of zombies being absolutely horrendous, making it start at 200 and rather moving towards a thousand or more. Even well developed bases will get problems then, at least if not defended properly with high caliber rifles, turrets and sturdy walls, fences and pits.
One of the most awesome events I ever had in Cataclysm was surviving a horde by setting wandering to true and city sizes to 8, my starting bunker got literally rolled over and broken down, even with fairly decent defenses around it, with me surprisingly surviving at the end. Had around 300 or so zombies rushing at me over the course of around half a day, with a nearby minefield gathering them on and on while they spotted some nice and living flesh close by with no way to retreat further away.
