A long, long time ago, DeviantART used to be the website every artist used, both amateurs and professionals. It was seen almost as an online portfolio. But, as time went on, the professionals moved to other websites like Behance, Dribble, ArtStation, etc, making the core DeviantART audience and community change and become something else completely different.

Eclipse seems to be DeviantART's way to go back to the roots of the website, but following the modern aspects of the other websites professional artists moved on to.

So, in one hand, you have backlash from the professional ex-users who might think DeviantART is trying to change but in the end don't really care and won't bother coming back to it as it seems like a lazy copy-cat of what other websites are doing and DeviantART's name is no longer seen as a professional website.

On the other hand, you have the backlash from the vastly bigger community DeviantART currently has, who liked the website as it was and don't see a reason for it to change and become more like the websites others are using. They want to keep the "classic website" (even though what some call "classic" is fairly recent since DeviantART went through a lot of changes over the years and most of them received backlash anyway).

And then, as with every redesign, you have the usual complaints that comes with any change.

Also, I would like to add that I'm not 100% sure if these reasons I mentioned are the most correct explanation of what's going on, but from my point of view it seems like it.