ATC on the training server is usually not very good and ATC on the expert server is almost nonexistent (we have to use schedules?!?!). Can IF put ATC on all servers and create a rating system for people to move up the ATC chain?
Unfortunately testing people for all servers is not practical, plus we want ATC to be accessible to all. The expert server has good quality controllers, so I would advise you fly there if you are wanting to fly under quality ATC.
There are a few groups that do do organised ATC on the training server, and many VAs have their own ATC section.
Let me know if you have any questions!