I've heard that some tournaments are easier to tackle compared to others. Is this true, and if so, which tournaments are easier and why?
This is true but the reasons why can vary. In respect to tournaments, generally provinces where light melee is the key unit are harder for both races as the losses tend to be higher than other tournaments.
Some people also claim provinces for elves where mages are the key unit are also harder, although I personally have not found these too bad.
People often ignore troops from the training ground and mercenary camp and this can influence how hard something might be. While getting the right unit type is critical, choosing the right unit within that type can have a large impact. For example 5 sword dancers verse 5 abbots will have a different outcome to 5 Cerberus v 5 abbots even though both would win in a balanced fight.
Regardless of this the numbers shown for catering verse auto-fight will still favour auto-fight even for these "harder" tournaments