The big benefit with JMAP is the state strings and /changes operation which allows even in the face of failures to recovery quite cleanly. It's really more a data synchronising protocol (RFC8620) with email support on top (RFC8621) and the new protocols coming in.
Section 3.10 of RFC8620 scares me. I know a lot about that and a lot of the complexity around that trite point is missing.
Note I've designed and built a complex messaging system that runs over HTTP over unreliable connections and run it in production for 15 years so I know what can and does go wrong.
Have you looked at their list of what doesn't work offline? It seems like it's mainly attachment management limitations than limitations of managing the emails themselves.