Sorry for being a noob but is this "tech" similar to the rails offline video linked or am I misunderstanding? I don't get why you would need a service instead of building it into the webapp/site?
Difficulties quickly come in if you want to: 1. Persist large amounts of data on the client. 2. Keep the data in sync incrementally (too much data to re-download every time). 3. Keep the data up-to-date in realtime (streaming changes). 4. Keep the data consistent, especially across multiple tables / types.
Many "offline" solutions are actually closer to caching rather than offline-first, which introduce all the issues associated with cache invalidation.
Also, if you're just working with a single table, you might get by with just using updated_at timestamps and soft deletes to be able to get incremental changes from the server. Even then you need to be careful with consistency issues, e.g. making sure timestamps are always in order and without duplicates. And if you start adding more tables, the complexity quickly increases.
The difficult parts are mostly related to keeping the local data in sync with the server, whether that uses SQLite, IndexedDB or some other database.