There is s certain amount of jealousy regarding Ubuntu's success, but I think one factor for the negativeness stems from things changing. People don't like change unless it benefits *them* and when there is change there is a lot of passion welling up.
One factor in Unity is timing ...
Unity came at a time when Gnome was changing things around with Gnome-shell (or Gnome 3). Some people may have felt they were getting the rug pulled out from under them because when Unity was rolled out, Gnome was switching to Gnome 3 and not doing any further development on Gnome 2. There was no "safe harbor", rather you either jumped one way, or the other and so people wanted to take out their anger, and directed it to whoever they could! Since Ubuntu was so popular, it had a large number of vocal people stating their unhappiness at the only target they know; Ubuntu/Unity.
At around the same time, there was what seemed like a series of spats (words..) between the Gnome project developers and Ubuntu/Canonical developers. Ubuntu pointed out how the Gnome project seemed to blatantly ignore Ubuntu's contributions while Gnome-shell looked eerily like the early versions of Unity. Naturally, people took "sides" and so everybody on the Gnome-side joined with the people complaining about Unity and started this whole Ubuntu-hate-fest.
Unity, and Gnome shell, has grown up since then. I still don't find either of them perfect but I do prefer Unity over Gnome shell today (check in with me tomorrow, I could change my opinion ) Other distros have tried to build Unity for their distribution but that hasn't gone very far and I think it has to do with incompatible libraries with Gnome or GTK making it "Gnome OR Unity". Since other distros have an established Gnome community they naturally will choose to stick with Gnome.
This gives people the feeling that Unity is Ubuntu-only, and is "closed" because only Ubuntu/Canonical has a say in its development.
Another thing was the "shopping lens". The shopping lens would not only search your local system, but search Amazon! This alone can questions ones privacy before taking into mind that this lens contacts Amazon through a Canonical server! Yes, this can be turned off but what it says is that Ubuntu is more interested in trying to make money than it is to provide a free (freedom) distro.
Not all people have forgotten when Ubuntu included Banshee as the default music player. The Banshee Project had a deal with Amazon music to provide an easy means to shop for music in Amazon, and Amazon would give the Banshee project a small amount from each sale. When Ubuntu included it as default, it muscled in and collected a portion of those revenues for themselves.
On top of that, things like Ubuntu's preference to "go it alone" with its own windows manager (Wayland? Mir? I don't remember) and make changes seemingly on its own gives people who don't know how to get involved, a feeling of it getting more and more closed. I'm sure somebody sees their mobile work to be another indicator of Ubuntu's movement to closed-source.
In the beginning, I think it was all about jealousy. Now that Ubuntu is doing more and more "on their own" it looks like Ubuntu is trying to become less and less "free" for the sake of making money. Arguments show up about Ubuntu not giving back to the community or passing changes upstream. I don't now if this is true or not or, as was the case with Gnome, the upstream was just ignoring their contributions.
Some of Ubuntu's moves have been brilliant, even if I don't agree with them initially or agree with their roll-out method, but I am nervous about their moves lately.
Disclaimer: This entire post is based on my readings online of blogs, posts and articles over the past 10 years of using Ubuntu. Some aspects, such as the technical pieces, could very well be incorrect and anybody should feel free to respectfully correct my view.