Isadora gets bilked by the way CON works in this game by lowering her AS, thus preventing her from doubling with weapons like the hand axe and reaver weapons and her strength isn’t enough to secure kills with iron weapons.She is also a prepromoted Paladin which doesn’t help your EXP rank and she’s subpar at fighting unlike Marcus, Hawkeye, Pent, etc.whi are all very strong prepromotes. While Marcus and co. may not help your EXP rank, they can at least take down opponents the rest of your units can’t which cant be said of Isadora.
Lucius uses light magic (worst magic type in the game) and has the same problem as Isadora in regards to weapon weight. He does promote to Bishop and get an instant C in Staves but promoting him eats into your funds and may dip into the exp as well.
I will concede that they both have small niches that keep them relevant, such as Isadora’s mobility and rescue utility and Lucius promoting to get an instant C Staff rank, but as far as competent units go; they are pretty bad.