you are welcome
The installation on the DC is "not recommended" due to high load caused by the collector. I cannot say that there are any issues or limitations but the collector uses resources to gather data from the VC, to publish it into WMI. SCOM agent itself also uses resources to process rules and monitors.
So we do not recommend to install collectors on any servers with specific and importent roles.
Enterprise Management service (EMS) - is the former Management Center (was renamed since the 5.5 version). This is the service which manages all collectors and monitoring jobs, performs load balancing and the HA. It is also the license server.
Any other questions please let me know,