To be honest, JAR actually worked very well - with the sole exception of the UK implementation of JAR-FCL which was a complete disaster.
JARs were "gentlemens agreements", nonetheless approved by ICAO, which were published across Europe then adopted within each country's national system as was considered acceptable locally. It was, with the one exception I've mentioned, a pretty good compromise between European standardisation and each control controlling it's own affairs.
I worked a lot with JAR airworthiness codes such as JAR-VLA and JAR-22 and whilst not perfect, they were a pretty good compromise which were fairly well understood and workable across Europe.
G