I was wondering the same. It does seem to have a lot of reviews and mostly positive on Steam, which is a good sign.
I guess it's one of the topics that I've seen discussed on here so many times, which is why it has piqued my curiosity.
There is a thread on here for the standalone version which is the one people are annoyed with, have a read and you'll see why.
To sum it up, it was well received when it came into early access 2 years ago and the majority of people were excited by it and pleased with what it was then and the development roadmap.
Now I've just checked and so far it has sold 3.4 million copies so far, a respectable number for any game.
The issues everyone had with it that the zombies were bugged and would clip through walls and floors, and that the games engine is inherently clunky, not to mention its relatively poor performance. Those were what I'd class as the main issues.
Instead of addressing stuff like that, the devs added new items of clothing into the game.
At this time there are 150 different types of hat to collect, the game is still clunky and the last time I played a month or so ago there hadn't been zombies for months, as they were so broken they just removed them from the game.
Playing it now to playing it 2 years ago, the main notable improvements is there is more rubbish to collect and thats about it.