One reason why the Law has been so popular is because it is simple to use. But simplicity is one thing, accuracy another. In the statistics section you will learn (if you didn't know it already) that at the table the Law predicts accurately no more than 35-37% of the time.
To us, the main defect with the Law is that it hides the real reasons why one side can take a certain number of tricks. It falsely assumes that there is a connection between tricks and trumps, but there isn't any. If total trumps and total tricks are equal on a certain deal, that is merely a coincidence – call it a side effect, if you like.
If you want to know exactly why there are a certain number of tricks on a deal, click on the link "What's important?" in the left frame.