Lowlight posted testing results from his courses some months ago.
I think they took them off the site because of the butthurt / people misinterpreting them.
But mainly most scopes are within 2%, roughly 50% were within 1% if I remember right.
Many Vortexes were tested and they had good track record to median 100%. ZCO was around 99-100. Few individual scopes from some mid grade makers were marked as failed / 25% error or not able to complete the test.
I have tested my scope with my ..capabilities.. and did not see anything wrong.
I believe that most usecases fall into:
1. Too much error -> scope goes back to manufacturer, no need to scale it
2. Not much error -> user does not even notice it
But to those cases where it is indeed needed, it is pretty much an unreplaceable tool.