There have indeed, historically, been cases where a union threw its weight around, bullied, and abused people.
However, a huge percentage of companies today are actively doing these things to their workers, and if you selected 10 companies at random where there were unionization efforts, I'd bet you money that in at least 9 of them, management was doing everything they could legally, and probably some things that are questionable or outright illegal, to try to prevent unionization.
Stop the "both sides"-ism. It's disingenuous and detrimental to genuine understanding of the situation.