Root canals have been round for hundreds of years, and are incredibly effective way to remove pain and to salvage the tooth structure. However, when poorly executed, can lead to some complications. The definition of poorly executed can vary a lot, but its not always possible to get the treatment 100% and this can lead to many health issues in the long run. One of the basic and many times overlooked and very important steps, is setting up a dental or rubber dam to isolate the tooth against saliva contamination. If this was not done, then the likelihood that things can go wrong is very high. Also, the use of sterilised instruments and making sure that all canals even the hidden ones have been properly treated and cleaned, for if not, the possibility of inflammation and even the formation of a cyst around the tooth in the long run is quite high. This is not good for systemic heath.
The use of CBCT to make sure there is no ongoing inflammation is imperative and we must remember that no treatment last forever. If you have systemic illness, you might want to look into alternatives however, I believe that went well performed by a trained Endodontist using state of the art protocols and instruments, it can be a very effective treatment to help patients with tooth ache as opposed to extraction, but must always be done under optimal conditions.
So, no! Not all root canals are bad, just the poorly executed ones. Always make informed decisions about your health.