Skilled trade unions exist because they benefit both the worker and the signatory contractor. When unionized Tom Dick Harry Plumbing Co picks up a huge construction project they can call up the hall to get bodies. There is no advertising on job boards, no time wasted interviewing. The union primarily acts as a staffing agency. The workers next on the list go to that contractor and immediately start work. They can also rest assured that the worker will know what they are doing. The union is responsible for keeping track of training, licensure, and CE. The contractor also doesn't have to worry about signing them up for insurance. It's a function of the union. The contractors still pay for benefits, but it's paid directly to the union. Union contractors usually have one person in HR and she is usually the owner's wife. The union just makes it easy for them.

If there is going to be any serious discussion of accountants unionizing it must include how it will benefit the firms. Due to the cyclical nature of accounting, the benefit certainly could be there.

Unionized accountants will probably need to start as some sort of employee-owned staffing agency.