It's somewhat stagnating, and nothing much seems to be happening. Nukes just hit, stuff is going wrong, but we can't see what's going on from their perspective.
And just zombies is getting a little repetitive, the same thing that happens with most zombie horror franchises.
Resident Evil (for example) manages to offset this fairly good by constantly throwing new mutations/monsters at the player. If they only introduced a few "evolved" zombies (faster, with claws growing out of their fingers or something), or adding some kind of stalking behemoth like Nemesis, it would go a long way to keep it interesting.
I am liking the character development though. Swordfighter-tan being a nutjob who gets wet by inflicting pain on others is awesome.