The rulers are definitely a shock to the system at first - they are more difficult than usual play for their difficulty, certainly. There are mods to make the rulers take normal turns instead of reactions, if you want to go that way, but I don't find them game-breaking at all (except maybe the Archon King, if you're wedded to low-damage runs and thus don't have backup soldiers). Appropriate use of the extra equipment will generally see you through - killing the Viper King in one encounter isn't much of a stretch.

This certainly adds more interest to the game, even if you already completed it.

The rulers are definitely a shock to the system at first - they are more difficult than usual play for their difficulty, certainly. There are mods to make the rulers take normal turns instead of reactions, if you want to go that way, but I don't find them game-breaking at all (except maybe the Archon King, if you're wedded to low-damage runs and thus don't have backup soldiers). Appropriate use of the extra equipment will generally see you through - killing the Viper King in one encounter isn't much of a stretch. This certainly adds more interest to the game, even if you already completed it.