I'd say Ryoma is pretty near untouchable, even on Lunatic. The solution to practically every problem Birthright throws at you is to just throw Ryoma at it and watch him mop up. Giving a unit like that a 2-range weapon was pretty needless, in my opinion. Xander is a decent tank with his huge def growth and personal weapon, but loses out due to his speed issues - he sure doesn't count as untouchable, if nearly everything can land a hit on him.
Though I agree that there are better overall units in other games, like Seth and Sigurd, but they also have other factors like availability over Ryoma or Xander.
I still preferred Birthright the most out of the three, though, personally. It was the most polished of them, I felt, and had something akin to difficulty scaling as you progress, while the other two are all over the place.
(finally, a discussion I can actually join in on!
)