IMO (FWIW) appraiser unions would erode the public trust in our independence.
As noted above, collective bargaining is the primary draw for a union. Organized work actions are just a tool the collective bargaining units use to collectively bargain.
Traditionally, the idea was for union members to compromise some of their individual interest - like being free to work for less or cross picket lines to the detriment of the group - in exchange for their interests as a member of the group.
If a group of workers is more loyal to their union than to their employer then the effects of that on their work can be profound. And although unions are often painted as working at the lowest common denominator, that doesn't necessarily have to be the case. The union's training programs can be - and reportedly are in some cases - superior in quality and expectations than what an employer might choose to hire on their own. It's not beyond the realm of possibility that a shop steward or other union functionary might have considerable influence on one of their slackers to straighten up and stop causing problems for the other workers.
With appraisers, we're supposed to be working in the best interests of our intended users, that best interest being interpreted as providing them with the unbiased, impartial and technically proficient workproduct that better enables them to make an informed decision. Whichever type of entity most effectively promulgates and *enforces* the expectations that lead to that result basically sets the standards for others to follow.
In the appraisal business I would say that it is the individuals themselves who currently wield the most influence on what they will and won't actually do in any given assignment, followed by some of the clients and fewer "other" intended users. IMO the state licensing boards have only minimal influence on what appraisers do and the professional orgs and other member groups have next-to-no influence on what their members do beyond their role of providing the technical training.
IMO, the most reasonable expectation we could have for the influence of an appraiser's union on appraisal quality would be similar to the "other member groups", who after all, are trying to build their membership and support and dues-paying revenues and who will probably not step on their donkeys to raise the level of play because they need every last member they can attract.