The data I've seen largely show that union membership is highly correlated with better pay, benefits, and working conditions. It seems clear to me that individuals are at an extreme disadvantage in wage/salary negotiations due to the enormous information asymmetry present when the counterparty is a corporation, and collective bargaining seems the most straightforward way to reduce that asymmetry.
As far as I'm aware, the reduced effectiveness of unions in the US is a direct consequence of union-busting legislation at both the state and federal level (e.g. so-called "right to work" laws, Reagan's actions against unionized/striking flight controllers). If that's the basis for your conclusion, I would agree in the sense that we need a strong political push to undo anti-labor legislation to ensure unions can be as effective as possible.