I've seen some people say that LIGHTSUM will end up like early CLC where they will be unsuccessful with the cute concept and will eventually switch to girl crush. Personally, I don't think this is true. CLC had a lot of reasons why the cute concept didn't work for them that LIGHTSUM doesn't have to worry about.
CLC debut in 2015, at that time the kpop market was oversaturated with cute concepts so CLC didn't stand out. There were also internal problems at CUBE such as CEO changes, 4minute disbanding, BEAST leaving CUBE, and lots of other fuckery behind the scenes. These resulted in terrible management decisions during CLC's rookie years.
Cube is at a much better place now than they were six years ago. Cube's management isn't perfect, heck, I'd barely call it good. But compared to back then LIGHTSUM is debuting with much stronger footing than CLC did. And right now, the popular concept among girl groups is girl crush. To the point where the market is becoming just as oversaturated as it was in 2015. By debuting LIGHTSUM with a cute concept, they will be put in contrast with a majority of the market and will stand out.
The only thing I would be worried about is the Soojin scandal, but it probably won't affect them that much.
TLDR: Cube's management and the kpop market are different now than they were during CLC's cute era, so LIGHTSUM probably won't have to deal with the same problems CLC did.
they will still be more successful than early 
