Virtually everything even slightly abstracted from directly quantifiable tasks: sporting skill, driving skill, intelligence, etc. None of those are measurable without significant confounding or a 'dumbing down' to some adjacent task (that still probably suffers from some degree of confounding via 'teaching to the test'). Sport performance depends on your cohort, driving accident rates depend on your environment, intelligence tests have a learned component, etc. Trying to develop a system that allows rank-ordering of ability along an axis that isn't subject to influence from an irrelevant external factor is impossible.