you should check out Phylol on youtube and let him show you how bad it is.
If we went by videos Phylol made of PBE content, we'd have to believe that Rageblade Vayne, Triforce Vayne, Rageblade Trist, and Hurricane Lucian are all overpowered builds. He loves to use new builds, and as a result he'll massively overhype unique builds, regardless of their actual strength. He can do this because he's good enough to make literally anything work against PBE opponents.
For the record, I've played Duskblade Vayne on the PBE. It's mediocre. You end up losing damage because you had to wait a full second in between autoattacks to make it work.
MF would be really good with it too. Cast E, AA (love tap, +duskblade, +sheen, +shiv) its a crap ton of burst for a single AA and when you put it on ANY adc with a skill they can activate first. Step into brush, you know its warded even if duskblades passive is down because you wont get the true damage proc after one second. If its not warded, pop out of bush and deal true damage. Wash rinse repeat.
Or, you could just keep hitting someone, and do more damage because you'reactually maximizing your dps. That also has the benefit of not forcing you to fight in an unwarded, bush heavy area and tanking your damage whenever you don't.
MF can use the build because she's extremely focused on ability damage, but that's not a bad thing. She's not like other AD carries, so there's no reason to force her into a cookie-cutter ADC build.