> a specialized engineering company based in, say, Portugal, that wins a contract for a major infrastructure project in Germany might find that German authorities require their engineers to undergo a lengthy "equivalence check’ despite holding qualifications recognized under EU frameworks."

This is not legal under EU law as far as I understand but EU countries are very good at turning a blind eye when that suits them.

The interesting and ironic bit that the UK was very good at implementing EU rules without trying to circumvent them that way and without adding red tape, which many supposedly pro-EU countries do, which is partly why the UK was so attractive (but also made people complain that they were "too honest").