Sharepoint 2013 - multiple authentication domains

We are in the process of rolling out a global portal in SharePoint 2013.    Currently we have an active directory domain we are connecting to.  One of our companies is using their own domain and don't want to move into ours.    
Are there any disadvantages of keeping them in their own domain and using ADFS for them to authenticate?   Is there any functionality they will lose within the Social realm of Sharepoint?  Has anyone seen this before?

Thanks

EmilyBen