Ubuntu is kind of the ugly duckling of the Linux world, but it's a very huge and non-ignorable ugly duckling. A good chunk of distros use it as a base, including the derivative I use: Linux Lite. I ran Ubuntu for 2 years before switching to anything else so it is probably the distro I know the most about, my second being Arch for 1 year after I ditched Ubuntu.
I remember the Unity desktop when Ubuntu switched to it in 2011 with version 11.04. At that time, I had no idea about the world of Linux, and I started with that distro in late April 2011, my very first taste ever of Linux. The articles that I remember reading, which are probably still published online somewhere, criticized the Unity desktop and why people wanted GNOME 2 back. I also had downloaded Fedora 15 "Lovelock" to test as well. It was one of the very FIRST EVER distros that came with the GNOME 3 desktop, when the GNOME project JUST had ditched version 2. Again, being new to Linux at that time didn't make a whole lot of sense trying to understand it all. The uproar of users retaliation against GNOME 3 has been around since I started in Linux. GNOME 3 is now alot worse than it was, removing the most basic features of a DE. Even XFCE has much more out of the box and it's probably a quarter of the size and RAM footprint vs. GNOME.
I loved the Unity desktop, and I still do. I loved it from day one, while most others realized Unity's potential around version 12.04 LTS. The DistroWatch reviewer, forgot his name, mentioned that Canonical "did their homework" and designed a DE that works very well for productivity, and I would agree. I am biased though, since I've used Ubuntu for 2 years before switching to ANYTHING.
To answer to why Ubuntu is hated, it's basically that people hate change. The Linux community is especially hostile to issues like this, which is why MATE exists now, due to the dissatisfaction of GNOME 3. Now that the GNOME developers are smoking crack and removing features from the DE by the day, it's high time to jump off that sinking ship and use KDE / XFCE / Insert DE or WM here. It's kinda sad now, and more sad that I didn't start Linux early enough to remember the good ole' days of GNOME 2 and why it was nearly universally liked. In fact I still don't understand what makes GNOME 2 so great and why people cared enough to continue to support it via the new branchoff called MATE.