Embedding fonts in an LRF book really, really
drastically slows down the page rendering - at least if you use the font for the "body" text of the book. It's fine if you only use it for titles, etc, but I'd forget about it for the body text. Page rendering goes from the usual half second to perhaps 3 or 4 seconds, which personally I find unacceptable.
If you want to see an example,
here is an LRF book I created a couple of years ago with an embedded Greek font. I find the speed way too slow, personally.