A while back someone or someones - too lazy to check - said that ZSS's Boost Kick or she in general is broken. In my opinion, I don't think anything she has really is broken. This includes her Uair chains to Boost Kick or her Flip Jump spike setups. The reason for the latter is kind of simple: there aren't a lot of phenomenal spikes. By that, I mean spikes you can setup at least in a 50/50 way or confirm it somehow. The only ones I can think of are like Luigi and Ryu's which granted, theirs are at most frame 10 while every other spike outside of Lucina, Marth, Palutena, the Pits, and Villager are like frame +14 and even those 6 aren't capable of setting them up as far as I'm aware. Spikes and Dairs are pretty lackluster in this game (and pretty much the entire series) with the best spikes being the fastest and that's like 2, having a special quality like ZSS's Flip Jump allowing her cover ground and high up, lingering and disjointed like Rosalina's, or Zelda's which spikes regardless which if she couldn't late spike, would probably put her Dair with the rest. For Dairs in general, outside of the fast Dairs and maybe Zelda, there's only like Dr. Mario?, Fox, Jigglypuff, Kirby, Lucario, Lucas, Mario, Meta Knight, Mii Swordfighter, Pac-Man?, Peach, and Wario's. They're all fast and actually usable outside of reads which pretty much all spikes are to the point where other spikes that are faster or have ways to be setup just lends to their ability to function. I mean, why does Wii Fit Trainer's frame 20 spike even exist when it's barely stronger than and has less active frames for both the clean hit and late hit than Falco's frame 16 spike? Even with her better (aerial) mobility, she's not going to be able to use that unless she knows exactly where you're going to be off-stage and that goes for pretty much everyone else. Ryu? He'll just Nair to Dair you. Luigi? Still can Dair you out of D-throw with the right DI. ZSS? Flip Jump setups. They have ways to setup their spikes and it doesn't really matter when they have better options to just outright kill you. As great as Captain Falcon's Dair is because of his speed allowing him to get right next to you, if you air dodge, I'd rather Knee you and kill you rather than dealing with a chance you'll make it back.
For Boost Kick... This has no actual evidence since I don't know how hit stun and blast zones interact. I do know however, that in static stages, you can jump as high as you want and not SD. It's only with the sides and the bottom where if you pass the blast zone, you die. So, let's assume this idea that if you're in hit stun after being hit and and past the blast zone at the top, you will die, but if you're not, then nothing happens. Now, Shuttle Loop hits twice where the first hit doesn't really move you much, but the second hit launches you up. Boost Kick for all loop hits move you up. The problem is that ZSS is the third highest jumper in the game and by moving up, she's getting even more vertical distance and with how Boost Kick works, she's also pushing you higher up and closer to the top. Other multi-hit Specials with odd hit angles like Mario's Super Jump Punch and Samus's Screw Attack don't kill you like that way up top for various reasons like outside of special circumstances, Mario and Samus won't be high up, or their Up Specials aren't as strong; for their last hits, Mario's does 3% with 52 base and 145 growth and Samus's does 1% with 50 base and 240 growth to ZSS's 4% with 50 base and 220 growth, but that can be considered moot since ZSS can kill with the loop hits which let's attribute to this idea of hit stun and blast zone equals death. This idea goes into that since ZSS's Boost Kick sometimes does not connect properly and you're currently not getting hit, you're in hit stun and you are considered "killable" if you pass a blast zone. This is made worse by the hitbox involving ZSS's extended foot basically tipping you into the blast zone. That's the problem: ZSS's Boost Kick failing to connect causes what I believe to be an unintentional side effect of killing weirdly and extremely early when it happens.
The fix is this: make Boost Kick connect better. If you've read this far, you're either thinking I'm a complete moron, a loon, or I'm a masochist and/or sadist. Read on and note the details. We fix Boost Kick by making all the loop hits auto-link, all loop hits have angles of 366 or 367 instead of vertical angles of 87 on hits 2 to 5, a random auto-link on hit 6, and a random 290 or 90 hit angle on hit 7, like Little Mac's Rising Uppercut, Peach's Parasol, and Roy's Blazer which all consists of a launcher for the first hit, the loop hits, and the finisher. The idea is to have you be considered attacked and not fall out or "up", be considered "killable", and die. Mario's Super Jump isn't really going to kill you unless you happen to fail a followup with a high juggle and he SJP's you to death; Mario can't jump that high - how ironic - and SJP's not that strong compared to Boost Kick. Also, SJP failing to completely connect doesn't cause issues with how Mario moves; he goes more diagonal as the loop hits have hit angles of 74 or 90 - assuming that 74's the one where it fully connects - than ZSS's almost vertical 87. Samus can't also jump that high and while she moves strictly up, Screw Attack's also not that strong and its hitbox is much more compact than ZSS's. So, this doesn't really fix the issue where ZSS can kill you early if she succeeds using a Boost Kick up top. The finisher can't really go any lower unless you want it to be a 0 degree hit angle that sends people strictly forward to its current 50 degree hit angle. You could lower the damage from 4% to whatever, but I don't think damage and hit angles are the problem; I think the problem is the knockback growth probably should drop to at least 150 if we're letting it connect better.
Also, if we want to complain about Up Specials, I think Ryu's should be the primary concern when he can hit confirm to Shoryuken and kill you at 70% on the ground. Neither Meta Knight or ZSS can do that and they have to confirm more than just one move to kill you up top while being near up top. In theory, an optimized Ryu just needs one Utilt or Dtilt to kill confirm you with Shoryuken while ZSS would need to somehow setup her Uair which needs to setup her Boost Kick. The closest to what Ryu can do is (pre-1.1.0) Fox who could jab to Up Smash, but that doesn't kill until 30% later and Fox is in a more vulnerable position than Ryu if he whiffs since Fox is much lighter and Up Smash isn't exactly safe whiffed not to mention only his feet are invincible for the clean hit while Ryu's completely invincible on startup.