I suspect that people think of getting a root canal as being the worst thing that can happen at a dentist because it used to be a lot more painful. Imagine having a living nerve removed without anesthesia and you can see why people would dread them.
But pain killers are so good now it's essentially the same as any other procedure at the dentist, and if it's done properly, it will probably stop hurting sooner than procedures like wisdom tooth removal.
If you have ongoing pain after a root canal it's likely that something went wrong. Very few dentists are perfect, so if you suspect that something has gone wrong, don't be afraid to speak up about it.