Can someone explain why root canal work is dangerous

My son of 24 has a cracked molar, and the dentist said that it will have to be checked every 6 months, at the moment nothing needs to be done but there may be a point when he will need to have root canal work done on it. It was only by chance that I knew of this. My son is a very independant responsible young man, but he has no idea about root canal fillings. I know that they are damaging, since when I had cancer a5 years ago I was told to get them out. I have since healed myself of stage 111 cancer - by natural means. The point here is that my kids are under some risk because I have had cancer, and I would like to be able to point out why in fact root canal fillings are not advisable. Is there anyone out there that can explain this clearly, because I just dont remember it all, just that it is not advisable.
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