There's one concept you have to get down and this game becomes a lot more tolerable...
The problem with the concept is this. It gets way boring and slows down the game. You can in fact kill the zombies but just firing away at them and they go down in 3-5 hits depending on your choice of weapon or their type.
Headshot and zombies are good for clearing out a group of zombies from afar, or when you've entered a new map or screen where they reside. Just trust me when I say, fire away, it's easier. Head shot when you can to put down zombies coming at you from a distance but for a quick run through a nearby group, or many approaching, just fire away.
Once you've embraced that concept, this game turns from being the Shemp of the series, to being much more tolerable. The side quests can get a bit repetitive at times, especially the typical escort missions but there is enough diversity in the game to get away from it.
I do feel that the game could use a better atmosphere break when you aren't in the quarantine area. To me, it just didn't seem like there was enough of a break from the dark of the quarantine zone and the rest of the city where the zombies had not gotten to.
Overall, I suggest taking the game in spurts, enjoying the head shots when you can but don't go overboard or you'll find yourself cursing the mechanics.
Bottom line, it's still Yakuza, just not quite there like Yakuza 3 or 4.