Unions don't always choose what is best for everyone. My aunt and uncle were working at a unionized automotive parts factory here in Canada and the factory needed to reduce payroll due to lost sales. They gave the union a choice of either cutting everyone's hours (and thus wages) by 10% or of laying off 10% of the workforce. When put to a vote, the choice was to lay off the 10%. This happens because when each union member votes, they know if they are in the 10% or not as layoffs are based on seniority. It wasn't some CEO taking a shortcut, it was the individual co-workers looking out for their own (perceived) best interests.
(I don't remember what the actual percentage was, but it was in the 10-20 range.)
"Unions don't always choose what is best for everyone."
Of course not, it depends highly on the ability of members of a group to colaborate. And that ability is on decline in the US since the mid-60s according to an extensive study in: (Book) Robert Putnam, "Bowling Alone".
In Germany they would usually vote against a layoff because they will not only think of themselves but of their co-workers and their families.
(I don't remember what the actual percentage was, but it was in the 10-20 range.)