We originally piloted using Nylas's APIs in our product about a year and a half ago. In that timespan, their pricing has changed significantly about 4 times and they do not seem like they have a particularly good grasp on their business model. They also, despite advertising 10 free test accounts, arbitrarily delete said accounts and then when you ask to get them reactivated you get connected with a sales rep.
Their API sync-engine [0] either hasn't been updated since March, or hasn't been kept open source--neither of which is a great sign. We're currently using the open source Nylas sync engine in production as we were uncomfortable trusting an important feature of our product in a company that doesn't seem to have a firm idea of what they want to do. But with the seemingly abandoned open source project, we are now working on building out our own syncing applications to ditch it altogether.
I would strongly caution anyone considering using the sync APIs to think about what they are getting into and the switching costs.