Three points:
1) Unions often don't include the most exploited workers - i.e. immigrants - and sometimes they are very hostile to them.
2) Whenever unions actually do form, they entrench, bureaucratize, and begin pursuing their own interests. This happens on a larger scale as well - for instance, in the Soviet Union, where the high-ranking officials had their own special shops, privileges, and so on, in the name of the authentic voice of the proletariat.
3) Workers in a factory have a common interest, and the idea of a union is to express that interest. But this idea of authenticity seems a bit tricky, since it suggests, as its reverse, inauthenticity, and so seem to involve someone out of it somehow deciding? I suspect that you only get "authentic" voices once they've stopped being authentic. Like, nobody is as Italian in Italy as they are in Little Italy New York...