Depends on the nature of the work and variability of worker skills, along with the union structure. For work where the skills of the workers are mostly consistent, or where skills can be easily grouped into buckets based on training and seniority, unions work out really good for the employees.

For highly variable skill work, and where skill levels are difficult to track (at least in a non-gameable way), you can have lower skilled (or lower performing) employees that drag down higher performers, due to union rules requiring things like seniority-based pay brackets. So people that are really good at convincing their management that they are really good, and work in industries where there is high competition for their specialty, those are the ones that tend to be more anti-union for their line of work.