I find she is pretty awful early game, but the best sage late-game. I used both Lugh and Lilina last PT.
Lugh was doing great in the first chapters, taking out hordes of enemies and DA'ing everybody. His magic and resistence however are sub-par for a mage. Res is important in this game because of all the enemy stave users, and his magic, while fine for most units, wasn't doing much against the tougher bosses. Lilina however, eventually gained enough magic to kill without DA'ing, and was dealing huge damage to the late-game bosses.
Against Zephial, Lugh did a paltry 9 damage, while Lilina did 22, with a 15 crit rate from Roy support. Lilina made my final team, and Lugh didn't. Though Lugh is probably better for longer, Lilina is a massive boon lategame.