It is very difficult to run into ruinous tax problems with publicly traded companies, but it's possible. There's a delay between the vest date and when you can actually sell your shares. Normally this is just a few days, but if something dire happens your stock can still tank in this window. If $100k of RSUs vest and then drop to $0 in value before you can sell, you have $100k of income to pay taxes on, and can only credit $3k of your capital losses per year towards normal income.
It is very easy to run into ruinous tax problems with privately traded companies. If you exercise options you have to pay taxes on the difference between the strike price and the current fair market value, and in exchange you've gotten something which is very difficult to sell. You can avoid ever having any tax problems by just never exercising your options until a liquidity event, but that often means letting them expire unexercised.