Yes, you have described the challenge in quantifying the most meaningful results.
My means of measuring improvements in sport form exist as measuring improvements in positional specific biodynamic speed/power/strength under the framework of alactic capacity.
Between what I measure via more quantifiable efforts and the feedback, regardless of its subjectivity, that I receive from the athletes- I know to a certainty that their motor potential necessary to heighten sport form has been increased.
The remainder of the idea is less certain as, per your reference, the extraordinarily predominant tactical component of the game, in addition to the team sport dynamic, creates a situation in which a heightened motor potential will not necessarily be realized as a measurable improvement in sport form if the statistically significance is not clear.
So between what I determine to train and measure and what the athletes tell me via subjective response- I possess, in my view, the most meaningful information.