We have worked on OfficeJs-based add-ins for the last 3 years and have delivered many projects to our clients. Currently, Outlook has a limited list of event-based features. Please think about a new event for ReceivedEmail.
There are many requirements that come from clients for their product that deals with new emails received in a mailbox but unfortunately, we can't do by office-based add-ins.
In VSTO, we can handle it.