If Rosalina uses it in the air, Luma sticks in place and is momentarily separated. Aerial repeat use is more limited that similar laser-style projectiles.

Reflecting the star bit always hits Luma as a result of that too. This is not true for similar projectiles done in the air.

It's not as fast as similar moves (in any regard), though the damage (5/4%) is quite good.

When Luma is separated, the coverage and stage control of the move becomes way worse.

Rosalina can't use at all it if Luma is in tumble or dead.

WORLDS

I agree that Speedy Star Bit is amazing, but it isn't quite old Falco laser. It feels overwhelming at first, but has some important limitations that make the move way more manageable with experience.Luma Warp is imo better than Luma Shot, but isn't amazing. It tends to get predictable, and does trivial damage--the combos out of it aren't reliable/consistent in my experience. Additionally, it tends to be a relatively risky separation. Luma is almost always vulnerable to being punted with an aerial out of Warp; it still must be used responsibly.Characters that get better customs for dealing with Rosalina, often specifically Luma:Fast Fireball outspams Speedy Star Bit, which he can safely reflect anyway. Explosive Jump Punch is probably scary for Rosalina, she dies vertically very easily.Bouncing Fireball gives him a reasonable harassment approach for Luma, and his Tornado. Clothesline Tornado could hypothetically be taken as a Luma killer.Side-b options might be better, but I strongly doubt this is a bad matchup for her to begin with.So there's this move called Dash Claw...Lick is pretty great against Luma, especially since Egg Lay isn't as good. Eggs also win trades with Speedy Star Bit.Koopa Drift (kart 2) and Big Mechakoopas destroy Luma. Hard. Piercing Cannon might be a plus too, but I can actually see him taking the default as a Luma killer too.Benefits little, but he does appreciate the faster bike to close the distance and recover that much better. Due to his aerial speed, he doesn't care as much about Speedy Star Bit as most.Short-Order Chef outspams Speedy Star Bit, which is a 42% (raw) Bucket anyway...Kong Cyclone melts Luma.Battering Banana Peel is way more effective if it hits Luma; normal Bananas do essentially nothing!Not sure if Power Bow would actually work in this matchup, but if it hits it EASILY OHKOs Luma. Piercing Arrows would also be better than the default here. Ripping Boomerang is great against Luma. Regular Boomerang probably better than default too. Also, he can hyrule shield Speedy Star Bit.Has a reflector for Speedy Star Bit, and all forms of Din's Fire are more viable traps on Luma than real characters. (No good in neutral though, naturally.) All forms of Phantom wreck Luma as well. Even if she chooses use defaults, Zelda is not bothered by Rosalina's upgrades in the slightest.I can't decide if Sheik would actually take Piercing Needles. Either way, needles still beat Speedy Star Bit, even if it almost events the score. Her default grenade is poor against Luma; both the others are really good though. Pisces (down 3, the long kick) might also be better in this matchup; Bouncing Fish normally only hits Luma or Rosalina, but Pisces tends to hit both.Holy crap. Wizard's Dropkick is incredible and beats Speedy Star Bit. His two command grabs, which Luma can punish, can be replaced by attacks that DESTROY Luma. Sheesh, evenstands a chance at actually hitting Luma!Not unlike Link, many of his disruptive projectile alternatives are a serious anti-Luma boon. Also, hyrule shield.While Speedy Star Bit is a pain for her, she more than ups the ante with her superior missile upgrades. Samus loves customs.Plasma Dash (side-b 2) is way more effective against Luma on hit than Plasma Whip, but probably not worth it. (Way less safe) However, Whip Lash is still more effective, and a reasonable option. Shooting Star Kick hits both targets better and might be interesting, but I'm also skeptical. Regardless, ZSS's speed and pressure mitigate a lot of the value of Speedy Star Bit and Luma Warp.Not only has a reflector to marginalize Speedy Star Bit (and side-b!), but with customs can take a way stronger one. Can take Piercing Arrow to deal with Luma, but I'm not sure it's worth it. If not, Guiding Arrow is pretty great and a better option for the context.LOL. Nevermind Reflect Barrier, Super Speed might be the best anti-Luma move in the game. Explosive Flame is good too. Lightweight is also really good in this matchup.Assault Dash is very valuable in this matchup.Close Combat!!!!!So Thoron+ is amazing against Luma (and Rosalina in general), but Speedy Star Bit can make it hard to charge. He might opt for Speed Thunder instead if he proves to have difficulty with Luma. However, Arcfire is already a decen anti-Luma move, and Arcfire+ (normally a poor choice) is an INCREDIBLE anti-Luma move that trades favorably with Speedy Star Bit.Copied Luma Shot is neat and seems to to trade favorably with Speedy Star Bit for what it's worth.Dedede Storm (neutral 2, the one with the repeating hitboxes) destroys Luma, often OHKOs. Good thing too, he hates Speedy Star Bit and Luma ruins Gordos...MK's main custom upgrade in general is Tornado 3 as a strong, moderate-risk vertical kill option. This is crucial against Rosalina! (Also, it will always kill Luma, which is a great consolation prize if it is blocked.) High-Speed Drill is also the superior anti-Luma side-b.Might consider Dash Counter to deal with Speedy Star Bit.Has a reflector to marginalize Speedy Star Bit, has the option to take a stronger (slower) one, and Twisting Fox (up 3) is a decent anti-Luma move.Void Reflector is amazing in general, and fantastic at killing Luma; who cares about reflecting star bits when you can do that! (It still blocks them)Discharge is probably a really good anti-Luma move.Dragon Rush is one of the best anti-Luma moves in the game. Rising Cyclone is a powerful vertical kill move, great against Rosalina.Ensnaring Aura Sphere is so much better against Luma. The long range Force Palm is probably good too.Doesn't care much about Speedy Star Bit or Luma Warp. Hyper Voice is trivially better than Sing, which is even more useless against Rosalina than ever before. (Hyper Voice actually kills Luma quite well!) Rollout is very poor because of Luma, so this is the only matchup where Jigglypuff should take Relentless Rollout. Pound Blitz is also possibly the superior option in this matchup.Rosalina is huge and floaty; this makes Zig-Zag Can amazing. Giant Gunman totally blocks Star Bits.Better options for spamming against Speedy Star Bit, and I recall one of the side-bs being a superior anti-Luma move? Reflects are good too...He can just absorb Speedy Star Bits, and all 3 PK Fires are uniquely great against Luma in their own way. The long-lasting one traps Luma for the entire duration, dooming it.Falcon Dash Punch, sort of like Warlock Blade is a lulzy and effective-yet-dubious anti-Luma nuclear option. Either custom Raptor Boost is important here, because the normal one cannot hit through Luma; both the armor/power of 2 or aerial mobility of 3 are welcome tools in this matchup. Default Falcon Kick is best though, as it's a good anti-Luma move.Everyone knows Timber Counter is crazy good. Not as amazing against Rosalina as many, but still good. And of course, Pocket beats Speedy Star Bit, unlike the normal star bits!Tackle Pikmin Throw is crucial for coping with Luma.JUMBO HOOP, DESTROYER OFLUMA.Same as Mario, but might consider Clothesline Tornado for annihilating Luma.If Speedy Star Bits prove really problematic, he can always take Dash Vision. But otherwise, Power Vision is such a huge upgrade on its own, and punts Luma pretty far. Armored Back Slash is the ultimate counter for separated Luma attacking you in the middle.No idea if his alternate fruit sets are better in the matchup; so hard to analyze. His Enticing Power Pellet kills Luma really easily, but I dunno if he'd actually take that.Skull Barrier is a functional reflector for Speedy Star Bit. Ice Slasher might be worth taking, but YMMV.Hammer Spin Dash didn't stop being crazy good or anything. It also helps avoid/beat Speedy Star Bit.Up-b 3i just great against Rosalina in general. This other custom options are way improved at killing Luma too, lots of the meaty multi-hit stuff that Luma hates.Customs let him take a reflector--also has superior anti-Luma moves.Has a reflector and decent anti-Luma tools, get way better with customs anyway. (Though less against Rosalina than most, ironically.)So yeah, those are the characters that benefit from having customs enabled against Rosalina in some way.I hope I didn't forget anybody.