You don't necessarily get taxable income from RSUs [edit: when they're granted] if they're structured in a 'Facebook Style' RSU which has an expiration if the company does not IPO within a certain time frame.

As long as there's a real risk of loss you can avoid the taxable income and private companies with high valuations will do this to help employees avoid the bad tax situation on an illiquid asset (without need to have huge amounts of cash to exercise options).

Options similarly must expire after 10 years for similar risk of loss tax reasons (as I understand it).

I was told the RSUs are 'Facebook Style' because they were the first to pioneer this.

Even options with a low strike price can be problematic because tax law is dumb and charges tax on the spread before sale when you exercise (so you can end up with a huge tax bill on exercise without the ability to sell the shares to cover it). ISOs were supposed to prevent this, but AMT has not increased to match inflation over time and was never updated to accommodate for this case specifically so you still have to pay tax if you hit it (which you will because it's low). This wasn't considered originally because companies intending to IPO were not private >10yrs so expiration risk was not a serious problem and you could just wait for the IPO before exercise.

So with options even if you save the exercise cash you have to save a large amount for taxes depending on the spread, or deal with a bunch of loan shark like companies that will take a cut to front you the capital.

For most people RSUs are probably preferable unless you get in really early and can exercise all the options when the spread is zero (preferably with an 83b election for early exercise on non-vested shares).