I've seen what large unions can do (just search for some of the largest Mexican unions, like the PEMEX one). Any organization of a certain scale and power gets corrupt, including private companies and unions.

I strongly believe in unions, but I agree with OP that keeping them small and focused is better for both the interests of the company and the employees. Extreme power imbalances are seldom (and I'd dare say never) good. I've seen unions effectively bankrupt business during tough times. I've also seen what unchecked corporate power can do. The right mixture of regulation and collective bargaining is a good thing, but like always, too much of a good thing can quickly turn bad. As an example, you can end up overprotecting unproductive or counter productive employees, reducing bonuses and pay for the other hard working employees because of reduced profitability.