In RFC4, some combinations of civs and strategies/victory conditions were too easy for me on Monarch once I knew how to achieve them, but too hard on Emperor. I think more difficulty levels, or sliders to customize difficulty, would have added more replayability.
In RFC5 some things were too easy on the hardest level (for example, doing anything as England after surviving the Viking invasions). But I attributed that more to limitations of the base game (it's less flexible / rich / expressive than Civ 4) and to the immaturity of the mod (for example, conqueror armies could be OP) than to not having enough difficulty levels.