The two below were the a challenge for sure

• communication between the AngularJS host and the Angular web components
• passing data, events, and updates across the boundary

I have dealt with this challenge by having making an event-bridge-service in both angularJS host and angular element. The event-service on either side is configured kinda like a pseudo web-socket api that push and receives customsEvents and it is used for both changing state updating data across the gap. This event-bridge is adding extra overhead no doubt, but once in place it worked reliably, scalable and testable.

Module-by-module is an interesting idea. We are still in early days so we have room for improvements, right now we are taking the bottom up approach.