I feel like people really overestimate the need to kill Luma. Yes, it needs to be done, but Rosalina doesn't turn into a pile of jello you can beat like a punching bag when Luma's gone. Her killing potential is toned down and she can more easily be grabbed. And that's it. Rosa players will almost always jump/roll away until Luma's gone and grab their opponents if they finally get within range (see: any rosalina tournament play).
That's what I find most frustrating about Rosalina. We can easily kill Luma with dash attack or fsmash, yea. But then what? We play keep away until she respawns. It could totally be a l2p thing and I sound like a baby. But I really think people overestimate the value of KOing Luma and underestimate solo Rosa.
(Also to be clear I definitely don't feel like Rosa needs adjustments compared to Sheik. She's just WAY more frustrating for VERY DIFFERENT reasons ;^