Unions are organizations that transfer value from employer and non-union members (including not-employed-yet employees and customers) to union members.
Long term effects of that transfer are rather unpredictable, because there are too many factors in economy.
No they aren't, they are organizations that utilize collective bargaining in place of productivity or efficiency increases in the labor force, thereby diluting the labor market with an artificial baseline at levels lower than the market would dictate otherwise. They don't transfer any value, they just reduce the overall value of the industry and provide the illusion that union members are receiving a satisfactory portion of this reduced value.