Though I should probably also mention that usability, by necessity, targets the broadest range of users. You've seem a lot of complaints about Ubuntu's UI, but it's about as simple as it can get. All the most common things are lined up neatly on the left.
No, that's not what usability is about. Usability is effectively and simply communicating a mental model to the user that enables them to feel "in-control" and allows them to do what they want in the way they expect. Different user environments have different usability requirements based upon frequency of use, average time of use and size and makeup of the expected user base.
Unity (and gtk3) removed a lot of useful functionality and is less stable than what it replaced and still hasn't caught up several years later. Those are real reasons to complain. My HCI prof founded that field with a study that proved that a specific known text interface was superior to a new GUI one for telephone operators. The reason for this was mostly a lack of keyboard shortcuts coupled with a known user-base that has a long average time of use, which is exactly what Ubuntu removed/changed and messed up with Unity.
Protip, if engineers can pick apart your UI design and your target market is engineers, you are doing it wrong...