Stock options have a tax advantage because they are taxed when you exercise your option. RSUs, however, are taxed at the time they are vested, not when you sell.
As RSUs grew more popular over the past five years or so, we've seen a problem emerging with how they're handled. Too many recipients insist on holding on to their RSUs, even after they vest. In doing so, they are falling into the trap of concentration risk—otherwise known as putting all your eggs in one basket.
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In and of themselves, RSUs are a good, solid equity compensation vehicle. An RSU is a grant valued in terms of company stock, but company stock is not issued at the time of the grant. Once the units vest, the company distributes shares, or sometimes cash, equal to the their value. Unlike stock options, which are worthless if share prices dip below the option price, RSUs maintain an intrinsic value unless your company goes out of business.
The challenge with RSUs grows out of how to use them. For most recipients, the correct approach is to cash the units out once they vest and then use the proceeds to build a diversified investment portfolio.