Scissors are the most complex hand motion to make, it requires enough dexterity to use two fingers and none else, probably the hardest motion for a baby to pull off. When playing in a roshambo tournament with babies scissors can crush the paper-based metagame.
Scissors are only weak in the sense that they are the only motion that is construct of society, and not a simple material. When the two blades are separated they aren't scissors anymore, they're just letter openers.
Fist Fight Strategy
When looking at all options of rock paper scissors, scissors is obviously least viable when put in a fist fight scenerio. To use the scissors hand motion in a fist fight effectively it should be used to gouge eyes in a 3 Stooges manner. While situational, this strategy can determine the winner quickly.