When I was playing my friend and he was falcon, as long as I didn't miss time or miss an L cancel, I could do double SH dairs to fsmash from hitting him off of his bounces on the ground at low percents, like 0-20%, and he get up to 55-75% really quickly.
It's true he could have teched either of those landings (and he's actually pretty good at teching), but he didn't, not at low percents that is. So that leads me to believe that the combo happens so fast, and the window for those techs is so small, that you have a good chance of racking up some serious percent really quickly (and possibly demoralizing them a bit haha) with this combo.
Since this is more a string rather than a true combo, I went ahead and did some testing on computers since they tech pretty much everything.
For fast fallers:
I found that if you want to get them in the hit stun of the dair, you can only do 1. The second one just is too slow and the lag you incur is longer than the time it takes them to hit the ground. If you want both they gotta be at like 80-90%, but then their DI makes it messy so it's an on the fly call.
For falcon, you can start getting a guaranteed dair > fsmash at 50%
Fox you can start it at more 70%
Falco, if you have very precise timing, and get the fsmash asap after the dair, you can start at 55%, but in the heat of battle that's really tough, so 65% is when it's pretty reliable
For mediums:
I didn't do much testing, but I found that you can start at about 40+%. Anything under won't work, so watch out for that.
I found you could get Sheik at 25% which was weird. I couldn't tell if this was legit or if she got a brief stand up before the hit, implying a shield or an attack
For lights:
Starts at 0%, works til about 20% until they can just jump out of it lol.
So even though this combo works at low percents mainly, on small maps like Yoshi's Story, it can nab you a quick kill near the edges, which is fantastic imo.
And a side note, if you're really good at aiming this, if you hit them with the back swing on the dair, since it sends them a little bit higher, you can get the fsmash off at about 10ish% lower