Hard for me to imagine any cases where the union does a worse deal on your behalf than you did yourself. Maybe you're talking about only compensation? Unions bargain for more than just compensation: better healthcare, parental leave, more sick days, caps on hours worked, etc.

In aggregate unions exist to get a bigger share of a company's profits for its workers. (Instead of executives and investors)

But if you have examples, I'd love to read about them.