MusicXML is the way to go as an interchange format for this sort of thing. The others are irrelevant as ways to store a written composition so you can re-use it.

A lot of folkies seem to use those little what I call "slate pc's" for words and music at sessions, so it could be a welcome addition to their abilities if it's any good.

They're promising something different - tablets in sessions are essentially read-only, this is touted as a composition tool.

I could see it suiting some people very well, assuming they can make it work as well as promised. Being platform-dependent isn't that big a deal; if a piece of software is good enough people will buy whatever hardware it takes to run it.