Perhaps I'm quibbling, or perhaps what I'm about to say will resonate with a lot of readers.
I wouldn't say that things are unmeasurable. I'd just say that we haven't developed a method to measure many of the things that make a system great.
Furthermore, it is a common error to conclude or at least assume that the common parameters that are measured, reported, and published are indeed the proper things to measure to draw a conclusion on the sound quality of a system.
Now I'm not saying measurements are useless, you just have to use them for what they are intended. For example, you need to know the sensitivity of a speaker measured in dB/w-m to judge how many watts you amplification you'll need to drive it.
Jerry