Being interchangeable is literally why unions are formed. If you have some unique skill set that is valued, a union is unnecessary and would hold you back. If you’re happy being slotted in to whatever the next available position is based on your seniority with no regard to you as an individual, unions are great.

That’s why they’re mostly autoworkers or longshoremen and whatnot an not professionals, outside a few niches motivated by ideology.