Proponents of a fixed exchange rate system point out that a major drawback of a floating exchange rate is that?

It does lead to uncertainty -- especially if you are trying to forecast a transaction that might take place far into the future. But, of course, fixed exchange rates can be uncertain, too, since they cannot be sustained indefinitely in the face of market imbalances. And when fixed rates have to be reset, they often do so in large moves, instead of day to day gradually.