I have done both the JAR and the FAA IR, and I found the JAR IR a bit harder, mainly becuase it contains more elements. There is simply more attention to detail, more to know, more checks, calls, briefings and so on. As an example, in the US they don't use QNH/QFE, and flight levels are only used above 18k ft, so one less thing to think about.
On the other hand, when doing a JAR IR you'll know more about your flight before even taking off, such as where you will have your engine failure, the FAA test is not as structured, more surprises.
I flew plenty of NDB approaches in the US (partial panel and full panel) and did not find them much easer over there, so can't say that is much different.