[Banthum]

Because it means de-emphasizing individuals by merging them into a group with one unified set of values.

This necessarily means erasing the voices of people who hold minority positions in that group.

It has every potential to be another place where those with the most time to burn, the most anger, and the most refined ideological weaponry grab power over those with minority viewpoints or healthy self-doubt.

Unions are supposed to fight for the thing the workers definitely have in common - their direct economic interests. Lashing that union structure to entirely different political fights invites the above kinds of problems.