Lancelot and Narses, how good are they?

They have one of the best passive skill in the game Rapid March. This fact makes them potentially very strong. But only in case You are going to utilize it properly.
Rapid March is good as You can assign slow and fast units into the legion and its mobility will be calculated as the mobility of the fastest unit.
For example You add slow Elephant and fast War axe cavalry. Mobility of the Elephant won't negatively affect the mobility of the whole legion, means it will be 11, just like Your War axe cavalry mobility.
And what is mobility? Mobility influences the quantity of attacks You can have per turn.
So in order to utilize the advantage of the generals with Rapid March best legion composition is units with high mobility and/or units with skills that modify mobility.
Such units are: Ravenna crossbowman, Mamluks (repeated fire skill - basically could provide You with 1 additional mobility refund after ach attack), Woad Raider, Mongolian Raider (Mobilize skill - attack cost -1 mobility point). So stacking these bonuses will make the cost of attack lower and as soon as Your gen could have high base mobility due to Rapid March the quantity of attacks per turn these gens could provide makes them potentially very very strong.

UPD: Speaking specifically about Lancelot and Narses.
Narses is good as he has unique equipment but at the same time he is stuck with special unit which is not that good. And Lancelot has very good active skill Desperate Wager which povides good synergy with the Legion composed of units I mentioned earlier.