Alright, so regarding Luma blocking projectiles for Rosalina, it's not as cut and dry as is being implied in this thread, time and time again. There's a very deep meta underneath it all, and I'm excited to see how it develops as our players progress in each matchup. Unfortunately, if Brawl was any indicator of how quickly players not using Rosalina will progress, I fear Rosalina's Luma-meta for the Rosalina player will develop significantly faster than the counter-play attached to it... but that's another topic altogether.
When it comes to RosaLuma vs projectiles, there are a lot of variables that comes into play during each interaction and who comes out on top. It's a very sweeping over-generalization to say "Luma blocks projectiles, making them useless." Yes, Luma is an incredibly useful tool to deal with projectiles, but it's by no stretch a zero-sum game.
A more accurate representation of Luma vs projectiles looks more like the following:
Luma acts similarly to Olimar's pikmin (at least in brawl) in that she has her own priority while being attached to the character. Luma has her own hitbox, her own hurtboxes, her own HP, which drops over time when she's hit throughout the match, etc. And without Luma, Rosalina is considerably weaker. Her zone of threat without the inclusion of Luma's additional hitboxes are both awkward for the player and abusable by the opponent.
Luma can't shield, or airdodge, or roll despite the cute stance she takes when Rosalina shields. :3 She takes damage, and will die after 52 total damage. Blocked projectiles lower Luma's health, and have an indirect impact on the game. The aforementioned impact being when Luma is inevitably killed. Afterwards, Rosalina is quite vulnerable for 8 seconds.
Even though blocked, many projectiles will desynch her from Rosalina, leading to a small time frame during which Rosalina is more vulnerable.
Luma blocks projectiles often times, but she's also got a small hurtbox. Often times projectiles will fly by her and have the chance to hit Rosalina's larger frame. Potentially, this forces the player to either account for that chance to hit her by shielding/airdodging/etc anyways. However, with time Rosalina players will learn to more precisely shield projectiles using Luma or avoid the projectile altogether so that Luma isn't hurt. It's a skill in itself to increase the block/dodge rate to 100% through w/e means- whether that's by jumping as a projectile is launched, spacing correctly so that the projectile and Luma line up, using neutral B, etc. This projectile blocking micro-meta is something both players have access to. It's not going to be completely one-sided. Every time Rosalina is forced to account for something, her overall option count is decreased. Patterns and other windows of opportunity, as tight a punish window they might have, will be exploitable.