It's completely possible for a ZSS player to remove any "risk" involved in landing boost kick OOS. This is the case for any character's OOS option if the player knows they have a frame advantage, knows their move will connect, etc, there is no guessing involved. Something I argued over in the past (and something I was wrong about) was that Link's up b oos isn't really that good because "what if the Link player misses and is stuck in endlag"... If an opponent misspaces a move (sometimes even slightly), then the situation is easily reactable for the player in shield and they can be guaranteed a punish. If not, then they won't use the move. There never actually has to be guessing (or risk) involved, especially the more the player knows about frame data. The same incorrect argument over Link's up b oos is often applied to other up B's like Super Jump Punch, Screw Attack, and Boost Kick, that these options OOS are "overrated" because if the player misses, they will be punished. This argument completely hinges on the theory that the player will miss these moves, which is a scenario that can be avoided. A player can guarantee that options OOS will hit.
Still, a key difference between Witch Twist and Boost Kick oos is that Witch Twist can be used somewhat liberally in situations where it isn't guaranteed, and if the Bayo player guesses right and catches an extended hurtbox, a poor approach, an opponent jumping over them, then it works out well for her; if the Bayo guesses wrong, she can be punished but not very hard. Witch Twist oos is a flexible option even if shield itself is a constraining defensive option. This is in contrast to Boost Kick, Screw Attack, etc, where the use of the move is generally reserved to where it's guaranteed it will hit.
These are still some of the best oos options in the game. Boost Kick and Super Jump Punch are the only moves that combine that much speed, range, and kill power into an oos option. With Boost Kick's range and startup, basically any misspaced move on ZSS's shield is guaranteed to be punished, and this translates to kills at generously low percents.
There's a bit more to it than that, though. If ZSS catches an opponent at the edge of Boost Kick's first hit, then whether the move connects effectively becomes a 50:50. If ZSS wins the 50:50, then she deals 16 damage and potentially kills you (an example: in the picture Bowserboy posted, Marss won the 50:50 and killed ZeRo). If she loses, the opponent pops out of the move and flies above ZSS while in hitstun, which means that the opponent gains almost nothing off of winning the 50:50 other than not taking damage and not getting killed. The real ratio in play is reward:no reward. If ZSS catches you closer into the Boost Kick, the opponent is forced to take more hits of the move and thus DI becomes increasingly reactable until it's guaranteed.
If the ZSS has high rage (like 130+ or so), an opponent can DI away on the Boost Kick oos and: a) if the character is a lightweight, they are guaranteed to fly out of the move and won't be able to punish ZSS. b) if the character is a midweight, they are guaranteed to pop out of the move and will be able to kill ZSS (this happened at one point in the Marss ZeRo set) c) if the opponent is a heavyweight, the same 50:50 scenario as before applies. A ZSS with rage should rarely go for Boost Kick OOS.
Stylistically, Marss is very liberal with Boost Kick oos. He uses it more like a Bayo would use Witch Twist--guessing when an opponent will extend a hurtbox, pick the wrong defensive option--he uses it banking on guessing right and landing the move, whether the opponent is even at kill percent or not. He uses it even when he has enough rage that the move stops working as well, I guess hoping that the opponent won't react fast enough to DI the move away.
Through all of 2016, Nairo and Marss' playstyles were sort of converging, until around late 2016 to early 2017, Nairo and Marss both used grounded boost kick a lot and got probably at least a third of their kills with the move. I'd say around Frostbite their styles began to diverge again, and now Nairo uses Boost Kick pretty much when it's guaranteed, and Marss uses it more than he ever has.
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Here's an interesting thing I noticed about ZSS jank. For a really brief explanation, there are two ways an aerial Boost Kick can jank you--the variant where the ZSS connects Hit 1 from slightly below, and the variant where the ZSS bypasses Hit 2, connects from slightly below, and the opponent flies out of the middle hits. There are actually subvariants of the latter scenario though, where connecting with hits 2-5 kills earlier than connecting 2-4 < connecting 2-3 < grazing with hit 2 only. Each of these Boost Kick variations changes naturally with how far the preceding uair sends the opponent, so each of these variations has its own kill percent range (I was interested in this because I kept seeing Sheik's dying to non-rage jank on normal ceiling stages, not just T&C).
Basically, even with 0 rage, ZSS has an almost continuous percent range where she can kill Sheik off of a grab. For an idea of what I mean, here are the rough kill percent ranges for each variant on FD (no rage), and the percent ranges listed where a kill isn't posssible is in parentheses:
28-31% -- Kill variant 1
(31-34%) -- The second uair of the ladder can only combo into a third uair, not up b.
(34-42%) -- Traditional ladder combo connects for 29%
40-43% -- Kill variant 2.1
46-47% -- Kill variant 2.2
49-54% -- Kill variant 2.3
58-61% -- Kill variant 2.4
Then beyond that, percent ranges for dthrow bair and uair.
This is why sometimes a ZSS will seemingly randomly kill a character off the top where you wouldn't expect her to, and with variations of DI from the opponent shifting percent ranges up and down and the percent ranges weird gaps, a grab on Sheik on FD from ~40-60+/-5 (and the percent range for Kill variant 1) potentially but pretty much randomly (scenarios that it would be crazy hard for either the ZSS or Sheik to account for) can lead to kills.
However, the closer the Sheik is relative to the ceiling when she gets grabbed makes these patches of percent ranges completely continuous--the gaps are no longer there at all. This means getting grabbed on a platform, getting grabbed when Lylat is tilted up, and getting grabbed on T&C period during this range will certainly lead to death. This is true for at least M2, Rosa, and Bayo (when she doesn't fall out lol), and I've seen it happen to other characters like Diddy as well (it would definitely be a much lower probability of happening to a character with Diddy physics however because the above kill percent ranges would narrow further).
Sheik ZSS is what a volatile MU really looks like.
's case, his lack of ground pokes is made up by his ridiculous sword and foxtrot which lets him weave in and out so easily, hence why people compared his foxtrot when he was released to a melee dash dance. If you watch Tweek's gameplay lately it mainly consists of a lot of empty movement with foxtrots, PPs and crossups
