Successful Guides tend to be niche and very specific. They address a very particular problem or subject and then provide a framework to the reader to overcome it.

So the more specific and niche the better, because you'll be able to make it actionable and create a unique overview that outlines exactly what the Guide covers and what results you'll achieve. Most writers, books, blogs and other media formats are pretty fuzzy on what you'll actually get out of them.

Guides on the other hand should be specific, no fluff, and actionable. If your subject needs to be tackled in several Guides, that's fine (and even recommended), but the more you can hone in to an exact problem and how to resolve it, the simpler the writing process will be.