The trade union, itself the product of industrial evolution, is subject to the laws of change, and the union that may have served some purpose a quarter of a century ago is now as completely out of date as the tools of industry that were then in use.
[...]
The old trade union is organized on the basis of the identity of interests of capitalists and wage workers, and spends its time and devotes its energies to harmonizing these two classes; and it is a vain and hopeless task. When these interests can be even temporarily harmonized it is always in the interest of the capitalist class, and at the expense of the working class.
Most capitalists heartily approve the old form of trade unionism and encourage and liberally support it, for the very reason that this outgrown unionism does not truly represent and cannot actually express the economic interests of the working class.
The simple fact is that industrial conditions have undergone such a complete change that now the trade union, instead of uniting the workers, divides them, incites craft jealousy, breeds dissension and promotes strife—the very things capitalists desire; for so long as the working class is divided, the capitalists will be secure in their dominion of the earth and the seas, and the millions of toilers will remain in subjection.
Quoting the great Eugene Debs:
https://www.marxists.org/archive/debs/works/1905/classunionism.htm