Q: Does the Windows Server 2012 Connection Broker balance incoming sessions based on processor and memory usage of existing sessions?
A: A farm of Remote Desktop Session Hosts is a collection of servers with the RDSH role configured--they operate as a collective service that users connect to via the Remote Desktop Connection Broker. The Connection Broker balances incoming requests for new sessions based on the number of sessions on each session host. New sessions are created on the session host with the least number of sessions.
The actual processor and memory utilization of each host isn't considered. The assumption is since the session's hosts are in a farm and offer the same services, the resource utilization should be fairly consistent barring any problems such as memory or processor leaks. A good way to mitigate problems with processes is to monitor the environment with a solution such as System Center Operations Manager.
Additionally in Windows Server 2012, by default Remote Desktop Session Hosts use a new fairshare process. This ensures the network, disk, and processor resources are fairly shared out between all users, ensuring no single user can adversely affect others by running very high loads in a session.
The fairshare capability replaces the Windows Server Resource Manager (WSRM) feature that was previously used and that has been deprecated in Windows Server 2012.