Well, I'm at chapter 22 in Birthright and some of them are quite hard this late, I barely made Kiragi's, don't think it was even possible for me to keep every unit alive, only 4 survived so I'm happy I don't play on classic.
Asugi's wasn't that bad, neither was Midori's or Shigure's.
I heard Shiro's is really hard late game so that's gonna be fun....
I have also heard people also have some trouble with Nina's but other than that I don't know.