To learn more about the features and limitations of a platform beyond the usual resource, reliability, and price questions, also ask about some features that are less well known but very important to maximize the store results:
Does the platform have native integration to Google AMP?
Does the platform provide social login on Facebook and Google?
Are all store pages accessed using HTTPS with HSTS?
Does the platform offer a free security certificate with auto-renewal for all store domains? (with and without www)
When was the last time the platform went offline and why did it happen?
In addition to these "difficult" questions, do not to neglect the detailed analysis of platform features.