This is a quick brain dump of problems I've run into using Rails Engines.
- Dependencies If an Engine requires a gem at version X, and the application requires the same gem at version Y. You could specify a lose version constraint on the Engine, but that's just asking for trouble. If the application updates to a newer version that the Engine isn't expecting, things go wrong.
- Layouts/Assets To customize these, you need to overwrite certain magical paths. This ends up duplicating a lot of the engine and forces you to dig into the internals of how the Engine works rather than trusting it as a standalone component.
- Models If you use an Engine's models, you're crossing into the Engine's abstractions again. It seems like a good place to share code, but it means your models will need to become versioned.
- Complexity It's hard to debug problems when they come up. Is it coming from the engine, the app, or the app's overrides of the engine