I strongly think DED should be part of Roy's ledge guard metagame (if I understand, metagame means the global playstyle of a character). It can kill enemies at low percentages.
when opponents are sent flying off the edge and want to recover, you can take a risk and jump off the stage and do a DED → ↑ x. This will spike them if you hit them with the 3rd hit of DED. If they are below the ledge to the point where you can't hit them with DED, you can Dtilt which usually has enough range to hit them, and then start your spike DED.
If your opponent is at about 20% damage, you should be fine to perform this amazing spike. If it's not close to that, say, plus or minus 20%, then you won't be able to perform it because anything less than 20%, the opponent will be to close and you'll probably miss, and if it's higher than 30%, your opponents can easily DI out of it.
EDIT: maybe I was exaggerating: I think the safety threshold is 20% to 80% or so, and thing more or less is not good.