It's situational, but I mean mostly in situations when using the Air Slash offensively will be your recovery at the same time. Like say you're falling diagonally down towards the stage ledge. Someone will chase out at you with F-air, or try to spike, but you air dodge past them and end up close below the ledge. You can then reverse Air Slash and hit them as you recover. It's just a really neat move to attack while recovering if you get the chance due to its massive sweeping hitbox. It can also work pretty well if you're high up in the air and can fall down onto the ledge afterwards, or if they try to stage spike you.
In the former situation falling below the ledge, Mighty Air Slash would work nicely with its increased vertical range, since you can delay the Up-B more to time it better as they're coming back on stage behind you. Advancing Air Slash just won't work off stage unless it's super high up, but theoretically it can be a very risky but rewarding game-ender if you use it off stage for the last kill since it has high side-KO potential
Oh, and that earlier video where Air Slash launched really high with just the first hit, I'm really not sure what happened. I tried to replicate it, but there doesn't seem to be any sort of special hitbox. From the video, I'm going to guess that maybe it could have been a weird wallbounce where AS hit them point blank on a wall, but it's hard to tell.