John from the app team here, thanks for the question! I'd say most of the unexpected challenges came from building an app on top of a system that was primarily designed for emails and web. There's lots of tools in the publish email pipeline (queues, delays, retries, etc) that we don't really need to worry about with an app where we're directly querying for the posts for a user. Conversely there are functions like push notifications that apply to the app but not email. This led to plenty of refactors and some parallel backend code, although for the most part we were able to take advantage of the rest of the company's work.

Separately there's been some interesting challenges on the client side around caching posts, serving post content offline, and purging spam/copyright infringing content.