I've started a NG+, going through with the new people, trying them all out at a low level, and...The characters are of varying worth in a low level party. Surprisingly, some of them are pretty good, and others...eghhh...
Shou - Tragically garbage. Hungry Tiger is somewhat okay, but it has a horrendous delay, and her lower-than-others base attack does her no favors for that. Radiant Treasure Gun is awful until you can start seriously out-gearing the trash she fights, Dazzling Gold is at least okay but still too expensive, Aura of Justice is hot trash, and she has both low starting MP and trash MP recovery. Her gimmick also doesn't really work in the first stratum, given the low length of the fights, and her lack of MP recovery means she needs to be enabled by items you don't get for a long time. Her affinities also undermine her bulk all the way up until the fire stratum She's honestly just completely outclassed by Suika and Momiji.
Mamizou - Good bulk, good damage, great base MP, at least some MP recovery (albiet not great), and can work without her gimimck, at least until she gets it (at a fairly reasonable point too, since she just needs 25 for the base, then 12 to upgrade it).
Futo - Her AoE is kind of miserable, but she's got the base formulas she needs to at least help out, especially in bosses. The plates gimmick doesn't really work all that well though and requires a truly silly amount of skill points to get going, so she starts lagging behind around the second stratum.
Miko - Miko is completely and utterly terrible in a NG+ run. Her affinities are insane, shes got overall really good resists, but her base stats are bad, her library cost and level up difficulty are tremendous, and while she doesnt have bad MP issues, her spells hit for a fairly low amount of damage. This is, of course, because her passives are incredible, but tremendously expensive. You can probably still get away with lugging her around, since it's LoT2, but...ugh. She's also locked into the two worst elements because ~of course she is~
Kokoro - Kokoro has the usual composite attacker issue of not really having gear until the third stratum, and the issues of her father of wanting a super expensive list of passives to function. She's also got really lackluster stats, but at least a good library cost. Also, her character synergy is - also extremely passive hungry, go figure.
Tokiko - Needs all her reading passives, and has exceptionally sub par damage, with extreme MP issues. She can fish for DTH at least.
Koishi - Extremely solid in a NG+ run, possessing good damage, decent enough base MP (albiet with terrible recovery without dumping 24 SP), and she's not really reliant on her gimmick either.
Akyuu - Completely worthless until you get subclasses. After that point she's a tabula rasa, but you'll probably just use her as a nuke. Not the best before you get her fully online, but decent.
Overall, the plus disc characters should probably only be used in NG+ if you really like them, as most of them were seemingly not designed with NG+ in mind for one reason or another, and are on the whole very skill point hungry. You can, to some extent, alleviate these woes for *one* character with training manuals, but that's about it.