Public companies are completely different from private companies though (by definition). In public companies, employees can liquidate vested shares immediately to cover any tax liability.

With RSUs in public companies, it's essentially (if not entirely) impossible to lose money. With RSUs in private companies, it's very much possible to be worse off than if you didn't have those RSUs to start with. Namely, if you pay the taxes out of pocket and the shares end up being worth less than what you paid in taxes (or are even completely worthless).

Options make the math even more complex regarding taxes/potential upside or downside.