I'm torn.
Reverend Descendant could lead to some good scenes where we appeal to the dead's sense of duty to punish their very... ah... descendants for their transgressions, but, hm, I think it could be a bit boring moment-to-moment.
Impudent Apostle is obviously fun.
Ecstatic Devotee probably means possession and possibility of body horror as we take on the traits of the dead (then again, as a Lunar we probably already have potential for body horror). Plus, literally becoming a vessel of vengeance and/or indulgence for ghosts is fun.
Matsu are Lions, and Lions are cool because they're the closest to actual samurai. Like, the thing about L5R is that it had to make the splats thematically different, which means it often reached for other archetypes than samurai themselves. So, Scorpions are samurai, but also kinda ninjas (except not as cool), Dragons are samurai, but also weird mountain mystics, Unicorns are samurai, but also semi-foreigners with their strange semi-foreign ways and exotic techniques like "Actually Aim Your Damn Bow" (it sounds better in Japanese). And so on.
Lions, on the other hand, are a lot closer to archetypal samurai. Their virtues are that of samurai: honor, courage, loyalty. Their vices, likewise: cruelty, arrogance, short temper.
Basically, they're the best clan if you want to engage with the concept of samurai.
That said, in our case they probably work better as enemies than allies.
Bayushi are, eh. They're Scorpion and tend to not be half as clever as they're supposed to be without failing to be twice as smug. I believe the author could do better job with them than the canon material, but still, fuck Scorpions.
Doji are fine. They're closer to courtiers than warriors, usually, but they do embody the ruling caste and what the empire stands for, so engaging with that could be interesting. Plus, them being more human than most ghosts should provide more opportunities for character interactions, positive or otherwise.
With all that said,
[X] Ecstatic Devotee Style. In the underworld, one cannot rely on honour or humility to placate the hungry dead, and so must resort to giving them what they want instead - the sensation of being alive.
[X] Doji. The leading light of the Crane, the Doji are trend setters and rule makers across Rokugan, ardent adherents of beauty in all its forms… even death. They are the most willing of the three to offer you advice and guidance, and the most 'human' in thought and desire, but their price is likely to be highly specific and esoteric.