Cities may go bankrupt because of private unions.
But that is only a small part of the tale. Detroit owes its public sector union workers lots and lots and lots of money they can never pay.
Detroit is broken because Democratic politicians offered up wonderful sweetheart deals to public unions. A private union can break an industry. It takes democratic politicians and public sector unions to really screw up whole cities, whole states, and whole nations.
Yes it was public sector unions and their lapdogs the Democratic party politicians. Plundering the taxpayer only works until the taxpayer realizes he is free to move. That is why Alabama is filling up.