Most Endos will give you an idea of what chance they give the tooth. If it's only a 20% chance do you really want to spend all that money? It's a decision only you can make, but generally, you would do well do be guided by your specialist.

I think what that person/dentist was trying to say is that the tooth had ONE extra CANAL. Yes, a front tooth only has one root, but occasionally they can have an extra canal. This is well-known by endodontists, but not so much be general dentists. In fact, it is pretty widely accepted now that upper molars often have four canals, whereas previously it was thought they usually only had three. Endodontists are trying to get this information out to general dentists, as it's a pretty common reason for RCT to fail. If you don't fill in every single canal it's so easy for bacteria to move into the root again, and cause infection. The extra canal is often very small, and difficult to locate, hence why they're mostly missed. Endodontists use specialist equipment, such as microscopes, to give them a better chance of successfully completing the RCT.

ETA I should say that general dentists usually do a great job of RCT - it's just not always simple, and it's great when they know when to refer a patient.