Very impressive stuff. I like!
As an aside, would these kind of illustrated covers work well only in certain genres? I am thinking they would be best for fantasy, sci-fi, etc. Or could an illustrated cover like this work well in other genres?
First of all, thanks!

About your question: I think that a good illustration, with a great aesthetic, usually works fine with virtually anything.
Certainly, it is of crucial importance to understand which style is the best for a particular work, based on its literary genre and the type of audience that will buy it.

For example, I really would like to illustrate a historical book, perhaps on ancient Greece! For this reason, obviously, I would tend to a "classic" style rather than a modern style.