I read about 70% of the Fedify book on the plane. It was carefully explained step by step, and I could easily understand the contents. Maybe it's because I have researched on implementing ActivityPub before, but I got the impression that I can create it easily. It is also good that there are plenty of ways to check how the implementation works. (I didn't know such a service existed…)
Fedify's only role is to speak ActivityPub, so you need to write your own UI and store data in a database (examples of each are included), but this also means that you can use any UI framework or relational database you like, which opens up a wide range of application possibilities. However, if Fedify does nothing too much, it will just be a hassle, so it is probably a good balance between the two. (Note: I'm saying this before I've even gotten around to doing it myself yet.)