There are a few important things that can go wrong in print and taking risks seemed a bit silly.
The first problem is that colours are always a bit off compared to what you see on your screen. For example the dark-grey cover background and the black details are not distinguished, unless you shine a light on it and stare hard. And the cloud highlights aren’t as bright as they look on your screen. Compare the picture above with the cover image up right.
Second, even for black and white, white and black are relative terms: what looks white on my screen turned up… well, look at the picture below.
The edge has been photoshopped off because it looked dirty but I didn’t realized how dirty it would have turned out in print.
The same can be said for printing colour images in black and white. In case you wonder Hexographer rivers and hills are extremely hard to tell apart in black and white.
The last concern is that interacting with a PDF and a physical book is very different. You can’t hold a PDF, use different fingers as bookmarks, it looks less in need of whitespace and, well, digital books can be searched more easily. It’s also easier to annotate physical books and, for some reason, the kerning in the printed cover looks much worse.
Nothing that can’t be fixed, of course. :)