We've been running Adagio with Server 2012 for almost a year now without issues. My understanding is that Server 2012 Essentials is just standard Server 2012 with a built-in 25-user limit (no need for client licenses.)

For a technical discussion see:
http://msmvps.com/blogs/bradley/archive/2013/10/02/comparing-standard-to-essentials.aspx