Besides the obvious stuff about it cannot be false/misleading, specific language includes things like "create false or unjustified expectations of favorable results," "imply the ability to influence any court, tribunal, regulatory agency, or similar body or official," "contain a representation that the member will perform specific professional services in current or future periods for a stated fee, estimated fee, or fee range when it was likely at the time of the representation that such fees would be substantially increased and the member failed to advise the prospective client of that likelihood," etc.
Our advertising has to abide by a conceptual framework, which is, of course, very subjective. We're also required to have safeguards in place to ensure our marketing is compliant, which would pose some issue with hiring a third party unless we could be sure the result would be compliant. I.e., we cannot say "Although the marketing was noncompliant, we hired a third party and we didn't know," because we have a due diligence type requirement which would ultimately make us responsible.
Part of the issue with marketing is, as you know, a big part of marketing is based around price/value. That poses a problem to CPA's, because we cannot bill on contingency or make guarantees, and while we can quote a person a certain rate and number of hours, we can't really guarantee that rate, because their could be overruns that we wouldn't know about in advance. Ultimately, we don't really know the final price of our services until after we've performed them (although we usually are within the quoted price at the engagement offset), so that all too poses a marketing difficulty.
As real life examples, we cannot guarantee a person won't be audited, if a person is already being audited we cannot guarantee the outcome of the audit, we cannot guarantee we can do anything whatsoever to reduce their tax liability besides producing a true and correct return, etc. Combined with it being difficult to market price/value, it becomes pretty difficult to really market a CPA's services, unless that CPA and/or firm has an industry specialty or something, and even then there are applicable rules regarding what representations can be made.
...and that's why you don't see a lot of CPA advertising besides a name and slogan on a park bench.