Using an abacus is not hard. Once you get used to the use of the abacus, you can use one just as well as a modern hand math tool.
For a very long time the abacus had been used instead of today's math tools. It was not until recent times that people made computers to make math tools. All the math that is in the world was mostly made and its thought helped by an abacus. Things like geometry and tough math questions were all made by some lines in the dirt and some stones.