It's tricky. But I will say, study study study! Do not slack on it. Most say if your test stops around 70 questions you should pass. But I had a friend who stopped at 72 and failed. It's a confidence test, testing your knowledge in the field.
Unsure on difficulty. It would depend on the state I would assume. I didn't think the NREMT was too difficult though. But I am also a pretty good test taker. I think MOST of the people who biff the NREMT know their stuff, but just get nervous. Just study your material, stay calm and you should do fine.
The NREMT isn't difficult. The testing methodology is what jams people up. It's a very different test taking set up than most people are used to. That why many of us suggest a CBT test prep service like JB learning to get you in the mindset of winning in the CBT arena.
However, there's lots of people that say "I was tops in the class, but failed the NREMT 3 times" and it's the NREMT's fault? Prepare yourself and you'll have zero issue.