Ah, I see. That said... basically you already nailed it. The actual number next to "level" isn't really relevant to me. I don't care what level my character is, I care how much hurt they emit & how much hurt they can take and how fast. The effect with Deis is comparable to a level penalty for me.
I didn't play Phantasy Star 2, but IIRC for whatever reason Nei's levels give her half the stats of other characters, but she gains them twice as fast. If hypothetically a Nei-like character joined halfway through the game at the exact same level as the party, she's not underleveled to casual examination, but in actuality she is - her stats are half as good as they should be for that level number. (Now what matters is how fast she catches up... the problem with say FE7 Nino, as you pointed out before, is that she doesn't really catch up fast enough without kill feeding her, so she deserves a level penalty for a really long time.)
Jo'ou: Well Deis1 joins more like "early Floor 3" I'd say at least, which is something. I do agree that characters whose stats don't scale easily throughout the game can be headachey.