well the ledge thing can be a little janky depending on the character because of differing ledge snap safety plus the stage you are on
like, i've fsmashed a ROB under the lip of smashville before, before he was able to grab the ledge. that prob wouldn't happen on duck hunt or something with no lip
i think you are right about the neck thing, because with the animation he swings his head forward and there's that whoosh animation line, that's what i am guessing is hitting under stages
you will probably never fsmash someone like sheik because when she snaps out of up b, her hurtbox is actually pretty far away from the stage, plus she doesn't physically come close to the ledge until she has already snapped it
but most characters have to bring their hurtboxes with them on the way to the ledge. and if their ledge snap size is smaller than your fsmash range is below stage, you can get them with slick ass timing. aforementioned ROB is pretty slow to get to the ledge + doesn't have hitboxes as he up Bs so you don't have to worry about getting hit before you release. little mac can only sweetspot the ledge after the last hit of his up b so he is pretty free to this too. wario's up b isn't super fast like mario's and i think he comes pretty close before he can actually sweetspot
but with most characters you will probably be banking on clipping them during that 1 frame ledgesnap vulnerability, which is risky with a slow + punishable move like fsmash
we can try and mess around with this because it would be nice to know the specifics of which characters and when and what stages. this was probably really good on lylat before the patch, might be even better now against people that purposely aim for the invisible lip underneath, since they will just be riding the stage for a bit right where fsmash could/should hit. plus dsmash similarly reaches really deep, not sure which smash hits further doe