It may or may not be in any individual's best interest.

For example, look at "bumping rights". If a company needs to eliminate a union position, and this is occupied by someone with say 20 years seniority, that person can "bump" some other union member out of their position who has a lower seniority. So, that person whose role was eliminated can push a person with only 5 years seniority out of their position. And then that person with 5 years seniority can bump a person with only one year seniority out of their position. And the person with 1 year seniority has no one newer than them so they get laid off.

Was it in the best interest of that newish employee to be part of a union? So they can act as a meat shield for someone much further in their career who would theoretically be much more employable in the general market?