From what I can see in the comments there is a risk of difficulty getting lowered in the later releases. This is mostly because people who find the game way too hard are more likely to be outspoken about it -- which is understandable.
However, as I see it the main issue is not the difficuly itself, but unpredictibility. While it's pretty easy to come up with a winning strategy and get to 100 zombies quickly, the 300 zombies goal was getting too frustrating and the two main issues to which I would attribute this both have to do with predicting opponents.
1. The main issue is visibility around map edges. The number of runs I lost to six+ berserkers just randomly rushing me, because game just happened to roll two large groups just near me as I was maneuvering away from danger... well, I wasn't exactly counting, but it happened enough times to be frustrating. I think the game would benefit from some way of warning about off-screen enemies so that when you *do* have to desperately escape and regroup you can at least have a decent guess which corner won't be a certain doom. If you want to make this a "dark souls of necromancy-themed mouse-driven games", you should also try to replicate what is actually the core of dark souls' difficulty -- punishing, but fair. As it is now Undead Craving doesn't give player enough information to really react in many situations.
2. Tied to the previous problem -- marksmen approaching from north or south can usually start shooting while they're still outside visible screen. I would say it's less of an issue than 1. because once you see bullets flying you actually *do* know they're there and you can start moving away from them, since they won't give chase. But as you move away from them you approach map's edge and that puts us back at point 1.
I just wanted to weigh in with my observations before difficulty nerf becomes a forgone conclusion.