It may be open source, but it's a thin client-side with your data being handled by the servers of a third party. No thanks, as trusting Google or FastMail with my email is a tough pill to swallow already, trusting an intermediary as well is way too much.
Looking through their API docs, one of the things that I noticed is that they added a transaction log ("delta" field), which IMAP doesn't have: https://www.nylas.com/docs/#deltas
Having a server communicate with IMAP (which is slow), cache the results, and export a more efficient/log-structured alternative probably makes syncing client computers much faster.
