Taking measurements will usually prompt questions of "how do I change the results over time?" So, although the difficulty of improving the thing doesn't change; the chance of taking action to improve the thing does change for the better.

Measurements will usually encourage attention and effort to change in a way that non-measured things are unlikely to.

On that basis, I'd argue that even things that are very difficult to measure quantitatively would benefit if a suitable proxy was found that could be measured.

Of course, you do have to be careful that your proxy is suitable!