I believe there is an issue with Spanish issued licences in that the Spanish equivalent of the CAA will not allow foreign (ie non Spanish) TREs to sign licence proficiency checks. At least one UK airline has therefore been requiring newly hired pilots with Spanish issued JAA licences to convert them to UK licences. Not a big problem but obviously involves some cost which in these days of low cost would be met by the licence holder. Still probably not that expensive compared to the savings to be made.