I can see a few tests for the bolded assumption that the numbers balance out in the end. First, if we take a hypothetical 4.0 book and compare it to a hypothetical 3.5 book, would it be safe to assume that the former is going to be better than the latter? No! Anyone who's been on NU for a while will know that certain genres and certain storytelling tacks will skew much higher than other ones will. We would first have to take into account what kind of books these are, and who is setting the ratings, and so on before making even a vague judgement on this. Next, a rating of 4.5 would normally be extremely hard to achieve. The way that statistics works means that you'd need seven 5s to offset each 1 to achieve such a result. Books that get rated this high should be the best of the best. There are several hundred works on NU that get this high, and theoretically every one of these should be at least a solid read. But should you expect to get a good book if you were to pick one of these at random? No! The above reason applies, but more than that, there's also other problems with books that get up there - things like books getting rated after only a handful of chapters, or books with very few ratings, or just really niche material that isn't exposed to a larger audience. And of course even books that do garner a larger audience tends to attract non-critical readers so the results end up on reflecting their tastes (or more accurately what they are enthusiastic about). Finally, from what I can tell, a lot of readers use ratings as a filter for books worth reading. So, is that going to generate good results for them? No! Or at least, they'll run face first into the wall of BL novels at the top and either embrace them or get turned off and assume that this is all that NU can offer. Now, those of us who are familiar with how ratings on NU work will know how bad an idea this is and can figure a way to navigate this mess, but it's a dreadful impediment for any newcomers. This also leads to all of those threads that go, "I've already read all the good books on NU, is that all there is?" The reality is that there are hundreds, if not thousands, of excellent books here, and the vast majority of them are obscured by the ratings system so they're next to impossible to find.
The amount of novels translated and linked in this site has grown rapidly that the standard for good or even decent novels has gone up. That and the amount of the novels with repetitive plot/character tropes/scenes being thrown here and there has made people very critical of novels. Also the expectations of the people here thinking that the quality of a said X-novel would be better or the same as the one they've read/currently reading.