Even if it was a Chopin topic I wouldn't answer. Like what do you want people to say? "Well, they're not easy!" Uh..well duh. No one here is going to give you an analysis of all 13 nocturnes. It's just a pointless question to ask really. Also difficulty ratings are stupid. Doesn't matter if it's Chopin, Faure, Liadov, Scriabin, or Mozart.
I was only hoping that one of you knowledgeable people on here could explain what the chief challenges are (so that I can assess whether I'm ready for them or not)
That's exactly what I meant by pointless though. Remember, these are 13 nocturnes composed in the span of 40 years. Pick the one you want to learn and then maybe ask the question again. Unless someone here has the patience to write out a response for each individual nocturne, you're just going to get the generic vague "requires good subtlety, nuance, and musicality" description which really isn't helpful at all.
They aren't Chopin nocturnes, that's for sure.