I'm more worried with them possibly becoming better bushi than... bushi.
Since you know, a Shugenja is pretty strong anyway, Right now the only thing they are losing compared to a Bushi(that im aware) is the speed that they will progress in ranks.
And then this is not a D&D game, where being at lvl 4 while your party is at lvl 7 is a problem. Challenge wont scale to you. Delaying ranks will be more an inconvenience than a deal breaker.(Which is why i like for other characters and at the same time is why i have mixed feelings about Shugenja with kata.)