RSUs have a strike price of $0 kind of by definition. And they're income when the restriction lapses, and that's complex when the stock is illiquid, so that's not ideal (sometimes, there's a dual restriction with time and liquidity, but that's messy as well)

Stock options could have a strike price of whatever, but tax favored stock options need to be granted at fair market value. Granting non-favored options away from FMV needs to be treated as immediate income, which is messy.