Yes, they are.
Buses are always available to DS because the Sassagoula boats don't have the capacity to handle all travelling between DS and the DVC resorts + PO.
Buses are available between WL & MK because the Motor Launches cannot accommodate wheelchairs, and also serve as an alternate mode of transportation between MK & FtW via connection.
OTOH, buses are not available between MK & the monorail resorts, nor between the Epcot resorts and Epcot or DHS. The monorails can take wheelchairs and have plenty of capacity, and the Motor Launches or walking are an available alternate mode. The Friendship Boats also take wheelchairs, and walking is the alternate mode.
In the case of the Skyliner, they will take wheelchairs and should have adequate capacity most if the time, but the only alternate mode would be buses. So it's open for debate.