Just because an ATC controller in Class C may not have ADSB capability to view the data does not negate the fact that 14CFR91.225(d)(1) requires the aircraft to have ADS-B out capability in that airspace.
I once checked in with approach control for VFR Flight following while approaching the Class B Mode C Veil. The controller knew the aircraft information right away. I asked if she was getting that off the ADS-B. She said, no, she just knew all the NxxxND callsigns from the local flight school. Then she issued me a traffic advisory for another aircraft that would be above me and "appeared to be circling". I spied the traffic visually, and then informed her that according to my ADSB-In data, that was another "ND" aircraft.
A lot of controllers may not display ADSB data if it is available, until they need it. Too much data on the screen increases the size of the data block and could cover another aircraft. Non controlled aircraft, just show the 1200 or 1202 and altitude, an "unverified" altitude that cannot be used for separation, "altitude shows 3600ft unverified".
