Yes it is definitely a tradeoff for the employee. For the employer, the cost of issuing RSUs I believe is tied with the fact that there would be no chance that the employee let go of their shares via not exercising their options (I do not believe RSUs are subject to employment tax).

But if you have 10 year exercise periods, that accomplishes the same thing for the employer's perspective. The difference them becomes the tradeoff for the employee between optionality and a possibly lower tax rate.