Well in my opinion, unions are the workers and vice-versa.
In a system of syndicalism that makes sense. Under capitalism, however, my concern is that firms tend to be like authoritarian regimes ruled by coalitions of owners, administrators, etc. with the unions meanwhile being narrowly-interested, disadvantaged opposition parties with little chance of (or will for) clambering up into power. This is often helpful to workers, true, but unions can be corrupt, they can be coercive in their treatment of workers, may at times betray the best interests of workers as workers themselves perceive them, and unions can be intolerant of competition from others even if it would ultimately be good for a lot of employees. I fear that unions do not advance the causes of socialism or cooperativization so much as they offer to imbue the capitalist system with some corporatist qualities.
I value unions - don't get me wrong here - but I do not trust them to speak on the behalf of all workers.