That combo is one of the key parts of Luigi's grab game in SSB4 IMO. Been using it myself pretty much since the day I started playing Luigi in SSB4 around a week after the 3DS release. It works on so many characters and hardly anybody tries to DI even at medium percents. Even when vectoring was around I still used this but some people were able to escape and with fair's then-decreased hitbox it wasn't as reliable as it is now.
I find it much harder to do on some characters than others. Sheik for example almost always manages to evade the follow-up grab (not CPU Sheik but human sheik) even at around 10-20%. I dunno if it's because she falls too fast or the guy I'm using it against has some incredible DI. It's also really hard/near impossible to do it on floaties like Jiggs/Kirby.
The good part is that no matter what, at least the first DThrow WILL lead into a fair or two even if you miss the followup grab.
Nice find on the first chaingrab tho. It's something I did like once and never bothered to do again since I assumed I was just lucky to hit the dair. Good work making it look very potent