I started dental practice in the seventies and it was not unusual to hear horror stories from patients about previous root canals. Many had teeth that turned black (from a silver disinfectant that is no longer used). Others had the root canal completed and lost the tooth anyway (this can still happen but it is much more rare now). Some recounted the pain and continued swelling over multiple appointments (In the past less was known about the desirability of getting the tooth cleaned out well early in treatment, eliminating bacteria cannot cause an abscess.) Even the old anesthetic was not as effective!
A modern root canal is very different. Dentists practicing today all had root canal training in dental school. Many endodontists (root canal specialists) are practicing today. Instrumentation and knowledge has improved immeasurably. The "R" word need no longer be feared.
This site will explain the nature of a root canal.