Via NBER, a new-ish paper on the union wage premium in 1950, at peak union power:
“The wage premium was larger at the bottom of the income distribution than at the middle or higher, larger for African Americans than for whites, and larger for those with low levels of education. Counterfactuals are consistent with the view that unions substantially narrowed urban wage inequality at mid-century.”
And, via Timothy Taylor, union density patterns in the OCED:
"1) Labor union power is weaker just about everywhere.
2) The extent of labor union power varies considerably across countries, many of which have roughly similar income levels. This pattern suggests that existence of unions, one way or another, may be less important for economic outcomes than the way in which those unions function. The chapter notes the importance of "peaceful and cooperative industrial relations," which can emerge--or not--from varying patterns of unionization."
Those peaceful and cooperative industrial relations are a big reason for Germany’s relative success in recent years. Relations were good, and so firms were able to ask for concessions from labor without too much backlash. The unemployment rate stayed low, so it's possible that in some dimensions this works quite well.
But it makes sense to think of those relations as a form of capital, or as a reservoir of goodwill: something that can be spent to achieve your ends (e.g., lowering labor costs), but not indefinitely. At some point, your reservoir's depleted, the capital spent, and your left with disgruntled workers who don’t trust "the powers that be." Germany's distinctive in that there’s both a cultural trope that saving/austerity/belt tightening is really crucial, and there’s enough social cohesion for people to believe everyone is tightening their belt and accepting pay cuts. That is, the cuts aren’t perceived as unfair.
In the long run, though, cooperative industrial relations are fragile; they are particularly hard to maintain when workers accept pay cuts. All it takes is for another group to do ok (e.g., immigrants / refugees, or the corporation / managers) for the setup to be perceived as unfair.