firstly, root canal treatments are not always successful, and the procedure is very technique sensitive---some are done well, some are not-- depends on who does it-- I personally am a general dentist who does many
root canals,
and I thinkmy post op films look better than those of a large percentage of root canal specialists.
filling the canals even a little past the apex can be a problem, especially on #3 or 14 due to the proximity of the sinus in some, but not all, patients.