Mario & Sonic Guy
Old rivalries live on!
Simply put, while Rosalina's recovery may be very large in length, it is flawed in that she's a slow faller, and Launch Star doesn't deal any damage at all. As such, Rosalina will often find herself having to stay above her opponent, and then fastfall if she expects to fully recover. Alternatively, Rosalina can just aim for a ledge to avoid any edge-guard attempts.To add on to icraq's post she's the 5th lightest character in the game while having a massive hurtbox.
She's also not that great at landing since she only really has her double jump and air dodge, while other characters such as Sheik, Diddy, Meta Knight, Pikachu, etc can use their other recovery moves to get back down to the ground
On a side note, Launch Star Attack can be used to damage opponents while recovering, but you are sacrificing travel distance, and Rosalina's aerial momentum takes a massive plummet when Launch Star Attack is finished. As such, it's much less effective for recovery than the standard Launch Star, which doesn't mess up Rosalina's aerial momentum.