It's actually less risky. The author may be able to reap the benefits of writing a book without actually finishing it. Ideally, maybe not much more than Chapter 1.
I'd say that I've finished all of my previous books, and I have no intention of doing anything different here. Of course, there's always the chance that I get run over by a bus or equivalent, but in that case, I assume that Manning would find a replacement (as per contract) who finishes the book. I don't think there are any benefits to be reaped from not finishing.
What refund? Nobody even mentioned a transaction taking place. The contents are on a github repo. Manning has zero risk here, except that if they keep getting burned by half finished books they may want to re-evaluate these contracts.
