I have multisite installed with buddypress.
On the main site all works 100% and I can view all the subdomain stats from admin panel.
The problem is when a subdomain user would like to view their stats, they can see the widgets but no stats.
I have created the sites in the piwik admin panel and in the users piwik setting I have added the id of the site for piwik counter, but no luck. And after I have added the extra sites in the piwik admin my site became very slow, locally.