Historically, I think it's true to say that most ICAO papers have been converted with an air law exam (to supposedly address the local airspace issues) and a flight test.
Logically, it's hard to argue with the above. The flight test is the ultimate check for somebody arriving with forged or otherwise bogus papers, which would be an obvious thing to try when going for a conversion route (and reading the airline pilot forums on here it seems to be common in certain parts of the world).
The problem is that even the air law stuff in Euroland is nearly all utter crap. Reportedly it was generated (excreted) by a Portugese ATCO and you get gems like this
which make a mockery of any pretence that this is being studied to make you a safe pilot in Europe.
Then, in the present system, for a working pilot, multiply the above by 14. To even get signed off to sit the exams (by the ground school FTO) you have to wade through some 3000 pages of this crap to do their homework. (CATS uses a better online system).
Even for the FCL008/CBM IR proposal, EASA did not have the resources to re-do any of the IR syllabus, so they took the existing Learning Objectives (syllabus) and chopped them down.
Can you really see EASA signing a bilateral with the FAA and comprehensively re-hashing the TK?
I like the comment about human factors
Here are a few more gems
