Problems arise if you are terminated in your employment (or quit) and you face a 90-day window to exercise options that will otherwise expire in a company that is promising but not established. For example, a former client of mine did this (after being terminated) in a solar startup in which he had been employed and got hit with a $600K tax bill. The company is all the rage among some VCs but completely unproven in the market and the stock itself has no liquidity and will not likely have any for years to come. If the company makes it big as hoped, this man will be rich; if not, he will have run the risk of a personal bankruptcy.
I would say that extremely risky situations occasionally arise involving employees and options but these are the exception and not the rule. In most cases, the risks are limited and manageable and the use of options is an excellent vehicle for the employees as a key financial incentive. The key, though, is to know what you are doing and to understand the risks before undertaking them. Beyond that, it is up to each individual and his own sense of risk tolerance.