Sorry, but based on your reviews, I don't fully understand what that category refers to, as some phones may have similar features on their hardware but are rated differently (I'm aware its different from Hardware Quality).
It refers to how the hardware was designed. Was it designed well? Is it comfortable to hold? Does it feel balanced? Are buttons placed and designed well? Are the various ports, slots, and covers easy to access?
It doesn't refer to the visual design, as that's a matter of personal taste. But it does refer to all of the practical aspects of the design.
There might seem to be some overlap with the Hardware Quality category, but that's more related to how well the design described above was executed. Hardware Usability is about how well the industrial designer did his or her job, while Hardware Quality is more about how well engineering and manufacturing did their jobs once the design was passed to them. ...