Problem with spells is that basically, each spell is worth a technique. So at each Rank, your Shugenja is gaining 3 techniques that often are more powerful and useful than "real" techniques, because "you need to expend resources to use them". This balancing factor is vastly overestimated and does...well...not much for balance.
To be fair, you're forgetting that spells require multiple rounds to cast (if higher than Rank 1), and can totally fail. If you miss your spell casting roll, you do nothing for the round except lose a spell slot (and in this edition, there are very few ways to boost your spell casting roll).
All of this on top of the (very) limited number of slots (vastly more limited in number than most other RPGs that use spell slots for casters).
Bushi/Courtiers have none of these drawbacks built into their techniques.
Hmmm... lightbulb. Wouldn't a quick and "dirty" way to fix the Shugenja and their powerlevel be increasing the casting time for some spells, particularly the more damaging/influential?
That would, on one hand, give time for the bushi to do their thing in combat (combat seems to be the main issue with the powerlevel discussion), and on the other hand, would force the shugenja to rely on a bushi to keep him/her protected during said casting time.
I'd also remove the option of calling raises to reduce the casting time, as that would be an easy-to-abuse loophole.
EDIT: I know they're different My point with that example was to show that the important thing was to keep the fun of all the players in the table.
Edited by Bayushi Karyudo