Random question, but Fox and Falco are the only ones with a general consensus advantage over Sheik.
As a Falco main, I personally believe Falco to have the disadvantage against Shiek.
As a Fox secondary, Fox wrecks Shiek's world. <_<
I personally believe Falco's disadvantageous matchups to be:
Marth. Falco, like Fox's disadvantage vs Marth is PURELY stage dependent. It's not the character specifically that wrecks Falco, but which stage Marth is on. Against Marth players, I frequently win on DL64, and Pokemon Stadium, whereas I get my world wrecked at FD, YS, and BF. FoD is neutral, since despite being a passive player, FoD opens up so many options for combos, as well as the platforms being more difficult for Marth to wreck Falco's world with.
Peach, depending ONLY how Falco approaches the matchup. An aggressive Falco will get demolished vs Peach, whereas a passive Falco will turn the matchup into a 50/50.
Jigglypuff, period. Jiggs is a **** hard match for Falco. =\
Shiek. This isn't something I can explain very well. Shiek just.. wins. Period. It's kinda gay. =\