The challenge is scale, if you aren't using a very flat and controlled (database like) rule system - so more like a knowledge store than a reasoning system - th

The challenge is scale, if you aren't using a very flat and controlled (database like) rule system - so more like a knowledge store than a reasoning system - then you run into big problems of complexity real fast. But even worse is the brittleness. Everything is fine, until it suddenly isn't and then you discover you can't touch anything without everything blowing up. On the other hand, if you have a system of case statements or if thens that is a bit opaque (or very opaque) refactoring them to use an expert system is a very good idea. And, if you have a homebrew knowledge store then using a rule system is also a good idea - but now-a-days you might want to try building a knowledge graph instead, mostly because that's v.trendy.