Maybe recently, with many American unions struggling just to survive, but historically (in the late 1800s to middle 1900s) American unions have been incredibly effective and pioneering.
Unions in the western world in the post-war period made historical compromises with industry that helped contribute to 30-40 years of relative labour peace and high middle class prosperity. They purged communists and other radicals from their ranks, took anti-Soviet positions, and adopted cooperative and conciliatory bargaining positions that gained higher pay and job security by dropping some of the more classic demands of shopfloor control and political change.
As a result unions started to look more like guilds than classic unions, they became managerial organizations like the businesses they were attached to, and they often lost legitimacy with their own memberships as a result.
When the economy started to contract in the 70s they weren't in a good position to fight, and their influence waned. And when the Soviet Union collapsed in the late 80s they lost any kind of bargaining position -- nobody was afraid of the reds anymore, so they didn't need the unions or social democrats or Keynesian liberals to pacify the population and stomp out communist sympathy.
