Modern web UI is nice, but at the end of the day when it comes to mobile, apps are still king.
If the purpose of this is to replace the interaction we had on Yammer, a mobile app is a necessity. Ease of use is critical, and navigating via browser does not give the features and interactivity that the Yammer app does. App specific notifications for replies, quickly navigating to favorite topics/groups. Just because it can be used in a mobile browser doesnt mean it should.
I see lack of a mobile app as a real barrier to winning user adoption and getting the most participation possible based on the proven success of Yammer Office 365 Network.