I love Billy and some of Under members are very charismatic, but sadly I don't believe Under can reach the status of those.

Undead's story advances too fast, and a member of Under died before he was even properly introduced.
Those other villain groups like Akatsuki reached their status because the groups lasted for years and years, and it also took at least two or three years before the first Espada or Akatsuki was defeated after the group's introduction.
yeah, all OG villian group from past had status from being 'long series', especially big 3..
but nowadays with middle long series (200+ ch) you need fast intro, and arc, like KNY, is the norm, they (mangaka of KNY,) slaughtered 4 lower moon villian (5-2) in 1 episode to reach Upper moon soon..
and to be fair, from UNDER, only RIP and SEAN are the one we know the most, others had some things missing from their characterization (ex, power, or backstory).. also author even still use RIP, as main villian after his arc, so what stop tozuka-sensei from using others member for recurring villian in arcs..
2 out of 11 main villian group, is still many.. and for Sean, he is JOBBER from his intro (get captured and humiliated), now maybe that its his gag, being jobber for others character.
They also had power to bring DEAD people to live,.. as long as Under had that, and Union cant stop them, they'll come back (in Ragnarock event)
LOOP... i dont know if loop will happen at this point (hope is still), and i dont know if we got another group or UNDER (again), who knows...
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Unburn, yeah, its actually had great potential:
a. if its self inserter, that mean he cant burn, anti-heat/anti fire, tank bomb?or explosion, great pilot for any robot that easily overheat. or become ICE (0 celcius-0 kelvin/absolute zero)
b. its external target, freezing. so yeah, its basically cool/cold power, made other lost heat, or any things/object or even environment lost their heat..
unburn is anti UMA burn (fire-base power), so unburn could be Ice/cold power in any shounen series..