It's not that hard if you have a good crew. Good crew is a very general term. Most good mechanics that know their ways around many automotive brands tends to think that they can run their own shop. Once they own their own shop, the nitty gritty of the business will no longer let him work on the cars. He then have to hire his own crew to work on the customer cars. The work quality will drop off and shit will start falling apart. If he smart he will hire someone to run the front office for him while he work on the cars, but that'll run into the $$$ problem.
If I was to run a performance shop I would stay with one brand of car that I know and know well, do good work, charge out of the wazoo to weed out the riff raft, and keep my customers and employees happy.