RSUs, as structured at companies I've been at, always have upside: you can sell them for the stock price, which is a least $0. At the worst, you make nothing. Tax is paid upon acquiring them by selling a number of the acquired RSUs to pay for the tax.
Options, however, can have downside: tax can't be paid with the excerised option itself, because you can't sell the exercised option. So you have to pay the tax out of pocket. Meanwhile, the company can go under, and render the options worthless: you've lost the tax amount. If the company's valuation increases significantly, the taxes can be fairly significant. But you also can't just wait to see if the company succeeds, either: every company I've been at forces you to exercise within a certain amount of time if you leave the company.
Options, however, can have downside: tax can't be paid with the excerised option itself, because you can't sell the exercised option. So you have to pay the tax out of pocket. Meanwhile, the company can go under, and render the options worthless: you've lost the tax amount. If the company's valuation increases significantly, the taxes can be fairly significant. But you also can't just wait to see if the company succeeds, either: every company I've been at forces you to exercise within a certain amount of time if you leave the company.