Myth would have been fine if Allm didn't make them unusable outside of Myth. Then they would have been simply endgame raid dungeons.
Rebirth was a dumb mechanic in every way. All it did was introduce artificial grinding content and make you grind the same stages over again for the SP and stats as incentives.
I really have no faith that this will be balanced because no Korean MMO is that well balanced, but hopefully they have enough sense to minimize the grinding so it'll be worth at least some shallow fun.