The prioritization would depend upon your requirements. We are trying hard to bring parity between the Microsoft Graph API and other earlier APIs. The O365 Management API was created earlier and is still used for multiple O365 operations. You may find that some Exchange online related operations are not similarly present in the current Graph API v1 just the way they are in O365 management APIs .
Please check out the Office 365 Usage Reports in Microsoft Graph API . You can get Outlook activity and mailbox usage details. I am not sure what your requirement is but you can mix and match these activity reports with the Outlook Mail REST API and obtain a lot of operational activity information for any particular user.
The beta version of the Microsoft Graph API will slowly unify all the services under one API umbrella and will be more extensive. While I encourage you to check it out , I would suggest not to use it in production . In case the information provided in this post helps , please do mark it as answer so that it is helpful for other users searching for similar queries.
We do not have a timeline on this as of now as to when we will have ability to get all kinds of exchange events as you see in O365 management API but you can track the change-log for the API and find any new changes as they happen. In case you have any suggestions , I would request you to post the same on Microsoft Graph Feature requests feedback site or up-vote if you find a similar request there. This is periodically reviewed and prioritized accordingly by the product group.
Thank you.