People really hated it because (IMHO) it had a really rocky launch. Ubuntu replaced a very usable GNOME 2.x desktop with an alpha-quality replacement. And Canonical was riding high from having "won" the Linux for desktop game and was pushing their weight around, giving the impression that they were ignoring the feedback from their users.
Unity wasn't without its missteps. But it matured quite nicely and I now prefer it to other desktop paradigms. Windows especially. It feels a bit stuck, like Microsoft thinks it reached peak desktop design in 1994. ;-)
Now that Ubuntu has ditched Unity, I really don't know what I'm going to do. I've been holding on running Ubuntu 16.04, but it's getting to be a burden. I don't like GNOME 3. I'm still not sure what to do. I might give Pop!_OS a decent kicking of the tires. I know that's still GNOME 3, but their take on it is the closest I've seen it get to tolerable.