After seeing Guzzlord's world backstory, I think all the UB homeworlds are post-apocalyptic earths (possibly Alolas) that met different ends.

Pheremosa desert = glassed by nukes (cockroaches survive)

Buzzwole jungle = global warming or volcano eruption heats the atmosphere

Celesteela = asteroid impact (craters all over its world)

Nihilego = flooding
stumped as to Kartana's though