Trade unions become very institutional and lose their original form and purpose as an association of workers. Instead it transforms into an organization to manage relations between management and labour. In the course of doing this, the legitimacy of the bosses is recognized, compromises will inevitably be made, and the union will cease to be a vehicle for revolutionary class struggle.
Instead we need to look to revolutionary industrial unionism, or syndicalism, for an example. Workers across all lines of specific profession organize themselves in solidarity for the purpose of building worker power, defending each other from the depredations of the bosses, winning immediate improvements, and eventually taking control of industry for themselves.
No more half measures. Worker freedom.