Won't disagree with you on the challenge of learning Mellel. I think the biggest is that they did not structure styles like MS Word. Thus, it makes sense in the Mellel way but for those coming from MS it can be a challenge. I had used MS Word since 1990, but the styles attracted me and once I caught on (Aha moment), it is so much easier and more stable than MS Word. Auto titles are another challenge in learning. Again, it almost requires an Aha moment.
NWP is more traditional in styles so easier to learn. I have found both much more reliable than Word or LibreOffice/OpenOffice. Development is a little faster on NWP.
In 2005 I wrote a book and tried to use MS Word, but after 50 pages it was not good. So I used NWP for a while, but it was not mature enough. I eventually tired Papyrus (German company), and used it all the way (including page layout with photos etc. to produce the files necessary for the printer to use for book production. Not one problem. Except they changed the program in 2007 and they still have not produced an English version of the software. I have written to them since 2008. But that is an entirely different story....