My hot take:
Tsubame-Gaeshi should have been dropped over Kaiten.
They both serve similar functions. One is used to enhance your Iaijutsu beforehand, the other is used to enhance your Iaijutsu afterward.
However, what they actually represent to the job are as different as night and day. Or should I say winter and summer.
Kaiten has a unique wind-up animation and a really cool name, translating as "turning of the heavens", fitting in with Samurai's theming around the shifting of the seasons--I cannot emphasize how well the artistic concept of Setsugekka aligns with and informs FFXIV's iteration of Samurai. Kaiten contributes *heavily* to Samurai's flavor as a patient/precise critical hitter (as well as an Mystic Knight expy job), literally describing and showing SAM waiting to strike. It provides a fair amount of job expression and decision-making, encouraging active Kenki management and theoretically being usable prior to any weaponskill, not just Iaijutsu.
Tsubame-Gaeshi is none of this. It's a quite lazily designed ability that just repeats the prior attack. It is a "Swallow Counter" that doesn't actually function as a counter, or imply any of the "swiftness" of Sasaki Kojiro's iconic namesake. It doesn't add anything to Samurai's core job fantasy of "wait, focus, and attack", and doesn't contribute quite as well to the Setsugekka motif as Kaiten does (swallows are kind of seasonal, but really only associated with spring and summer). As implemented it's actually quite antithetical to Samurai's job fantasy, being unsubtle "win-more" big-damage spectacle instead of anything suggesting grace, patience, or precision. And on top of that it doesn't have any of the player agency created by Kaiten and its interaction with the Kenki gauge or non-Iaijutsu abilities.
I felt the need to make this a separate thread because I haven't seen much discussion of what I consider the flip-side of Kaiten. Tsubame-Gaeshi is the worst ability in SAM's kit. Frankly it's a terrible ability all-around and if the only reason it exists is as rotational filler then it should have been removed to make room for better SAM abilities.