In any realistic scenario, those shares would quickly be diluted to nothing when the company needs to raise money from capital markets. Unions are in general a much better way to protect workers.

The real power disparity between capital and labor is that capital is concentrated and labor is diffuse. Every worker negotiates with the corporation as an individual over compensation and worker rights. When you have a union negotiating the contract, then both labor and capital are concentrated and on a more equal footing, producing more equitable outcomes. Giving employees a tiny ownership stake doesn't really change the power disparity at all.