Aside from opinion, the numbers from the government of Canada show that incomes were stagnant by the early 70s (which roughly corresponds with income stagnation seen in the US). This is again when unionization started to decline in the US, which could reasonably attributed to the decline of unionization. However, Canada saw strengthening unionization in the same period (peaking in the mid-80s). Based on what people claim unions accomplish, income stagnation is unexpected given how strong the unions were during that time.