If this were true we wouldn't have seen the explosive rise in the precariat. Another separator is what each lobbies for and on behalf of.
Corporations lobby exclusively for themselves and bottom line thus against the interests of the people. Unions on the other hand represent the working class and disenfranchised, lobbying for the betterment of the majority of society.
By lumping the two together what ends up happening is corporations continue to exert outsized influence on politicians and government while unions and the majority are effectively shut out of the process.
Corporations through being able to reward compliant pols with lucrative positions after they leave office have other avenues of influence not available to unions. When was the last time you read of a pol like Baird quitting to go work for a union.
But not content with this advantage corporations have learned how to game the system
SNC-Lavalin is defending $15,000 in political donations made during the 2011 federal election to a Conservative candidate by at least 12 company executives and two family membersThere are other examples of Conservative ridings with zero chance of winning receiving tens of thousands of dollars in similar donations only to see much of that transferred to other more important ridings.
So by all means lets get the influence of big money out of our system but keep in mind one type of money is a problem the other isn't Equating the two is wrong and only ends up causing more damage.
The amount of money from unions compared to corporate dollars is dwarfed by the power of infinity