Well she gets with her aw a 2% chance of assassinating an enemy and that's quite a lot for a unit that deals dmg to all units it blocks.
Sakuya is also not so bad, with aw she boosts Samurais and Ninjas attack for 7% by just having her in the team. What can be usefull, if you need a ninja to deal a good amount of damage. But that's still quite situational and in most times just a more-value setting.
So yeah no hurry in lvling them, but they can be nice.
I tried using a Samurai and I found they are too niche for my style of play. The only time they did much as if I couldn't get an archer in range due to limited ranged unit slots. And even then I wished I used a Soldier or 2 cheap single Duelists instead. Even a heavy does better a lot of the time and Samurai are about the same price to play.
Its a pity as the idea behind them is interesting but they are too expensive for what they do. The cheaper options are all better and versatile.
That's Shizuka, not Sakuya.And tbh, you need ninja(they're technically ranged rogue, due to 2/3 of them currently in Nutaku has 50% eva during skill) and after AW, ninja cost go down while Samurai go up, so unless you got Shizuka otherwise Samurai is better if you in need of blocker after your whole team has been deployed and your soldier has no other job to do.