Under the leadership of prof Hess, MPI Luxembourg have collated a treasure chest of data on what, in practice, continues to hold up recognition and enforcement of judgments in the EU Member States. The Study, released last week, was conducted for the European Commission. Its main conclusion suggests that in particular the service of documents could do with streamlining.
That all in all modest recommendation suggests that the very variety of civil procedure rules in the EU Member States in and of itself is not the main obstacle in recognition and enforcement. I insert a big caveat here for I have so far only read the exec summary and the main recommendations, however if they are anything to go by, the study in effect has to serve as a strong argument against more harmonisation of civil procedure rules at the EU level.
Debate no doubt to be continued.
(Handbook of) EU Private international law, 2nd ed. 2016, Chapter 2, Heading 2.2.16.