The U3 specifically does not include discouraged workers, those who are underemployed, or those who simply aren't looking.
As long as we are comparing apples to apples (i.e., these figures are all indicative of the same basic data with the same restrictions), there's no issue comparing them.
The "same restrictions" apply, but countries in the first category will have fewer "unemployed" people even if they have the same number of discouraged/underemployed/jobless/whatever people.
Is that apples to apples?
Are there enough people to whom that distinction applies to be more than statistical noise relative to the other regional variations that would have an impact?
Seems like a minor nit to pick in the grand scheme of things.
Here is a thing in English that seems to come for free if you give them bogus personal data: https://www.agenda-austria.at/en/publication/austria-the-lan...
A longer German blurb about this study is here: https://www.agenda-austria.at/oesterreich-das-land-der-verst...
They claim 250,000 hidden unemployed for 220,000 "official" unemployed for Q1 2013, i.e., the official figures would be off by a factor of more than 2. But they don't say how many of those hidden unemployed are in training courses. Maybe the full study does say, I can't be bothered.