Also, any state or federal agency with taxing authority can and sometimes will audit either companies or accounting firms. I went through a state level labor department audit. Their hook is that they collect unemployment insurance premiums, but that extends to anyone being paid by the company that was not taxed as wages - like contract employees and dividend recipients. I'm happy to say it went so slick the auditor wrapped it up in one day.
I don't know about rating CPA firms, either.