One key difference for unions is whether a job is project based or more of a permanent placement. The problems you mentioned are often a problem in the latter like with teacher or police unions. Project based workplaces are different in that they have a natural turnover and people need to be continually rehired so seniority or difficult to remove employees are less of a problem. The various unions in the entertainment industry are a good example of this. Video games are closer to the project based end of the spectrum.