The ongoing debate over Office 365 Groups and Yammer

select the right tool for the job. It seems to me that:

All Office 365 tenants can make use of email to stay connected internally and externally
Tenants that have a background in email and Outlook will find it natural to use Office 365 Groups as the basis for team collaboration up to a limit of say, a thousand users.
Some tenants will prefer SharePoint team sites, especially if their needs are document-centric.
The larger the tenant grows, the more useful Yammer can be in terms of hosting large, relatively unstructured, discussions covering everything from issues affecting the entire company to new product development. If you need to understand how effective Yammer is in these kind of discussions, join Microsoft's Office 365 IT Network to see how debates evolve (and to observe both good and bad user habits).