LOL like we didn't know this. but here's the disclaimers.
1) when the heck will they let you dair them on the ground
2) there's moves that can stop this. Shield, crouch cancel, grab, shadow dodge, wavedash, roll dodge, etc.
3) they can tech
4) the stun might not be enough to get a whole smash in.
5) if you miss, the lag will punish YOU
6) Roy has extremely low stun time
get it?
Rules of smash:
#1 Always L-cancel
#2 Always Tech
#3 Don't get hit
#4 No one is perfect, and thus nothing is impossible.
I do moves like this often, and it connects 80% of the time. It just requires that you have good timing and an open mind. If the situation where this is usable, then it's good to know. And sometimes, because this move is flawed, a person of a higher mind will not expect it and thus get hit by it. This is the phenomena of "Beginner's Luck". I think it's a perfectly usable move, but we probably have completely different playstyles.
And aside from that, you're far too venomous against this new host of enthusasim. With this being a situational move, that in itself means that it is not perfect and will be hard to execute. The situation he gave is a nice one, and one that would probably be better is, say:
An enemy just whiffed an aerial and is still in the air and the lag of the attack. If you're on your feet you can Full hop D-air fastfall l-cancel onto a platform, thus ending the enemy attack and bringing them onto the platform with Roy's crazy D-air, where you can f-smash with a grin on your face.
I'm sure that there are tons of better options, but to keep the opponent confused is a huge part of the game. For example in the same situation you can:
Shffl a fair or two into a d-tilt forward smash
Shffl a nair and follow with a full hop neutral b charge
Dash dance into grab>Dthrow f-smash
Dance cancelled Up smash
D-tilt into DED into edge guard
These are all viable responses to the situation I had provided, but to mix it up gives you flash, style, makes it less likely that they will find a way to seal your moves and punish you.
In short, what I am saying is: Every maneuver works, you just have to know when to apply them. The larger your move "Vocabulary", the better you will look, feel, and be. Because humans aren't perfect, that allows some of the coolest combos, situational or not, to be possible.