There are ways of dealing with them – just so long as it’s not Day 2 and you unassumingly take a 1-skull mission as your second contract and (still with a band of six fighters in cloth/leather, without high attack and defence skills and without high durability or weapon efficiency, and armed only with spears and bludgeons) find yourself up against a greater number of nachzehrers, including a ready-made giant one with auto-swallow ability. Under these circumstances, you may already be wounded and may be reluctant to move into melee with 2-3 of the beasts (who will act before you) so as to prevent them from consuming the corpse you just created (assuming you have any action points left after killing it). You may also be reluctant to hold off on dealing a killing blow with your final action points of the turn, since you may already be wounded and/or wavering and/or lacking the energy to form a protective shieldwall and/or concerned that enduring another round of attacks will leave you without enough energy to keep stabbing away at the giant one with your mighty spear-point (still less knock it out with a club). At this point in the game, arrows usually miss (but if they do hit, it’s bound to be an accidental hit against a blocking one that’s already wounded, and which will now provide the nutrition for another of them to attain auto-swallow ability) and spearwalls are exhausting (or at least would be if they didn’t get breached at the first or second attempt).
Once you have a full squad in decent armour with better stats and helpful perks, it all becomes much more manageable. Certainly, by that point, fighting nachzehrers is much more interesting than fighting ghouls in days of olde, but there does seem to be a bit of “imbalance” in how readily you can (sometimes, at random) become swarmed by their parties (sometimes including giant ones) in the very early days when there’s no way you could hope to deal with them.