Payloads have to withstand forces of several G, but those forces point backwards toward the rocket engines (or close to). Also, while the launcher is in the pad the satellite is subject to 1G, but again pointing down in the same direction relative to the payload adapter.
But for horizontal assembly, the satellite must be linked to the payload adapter by it's base, forming a cantilever that has to support it's own weight until the rocket is put upright prior to it's launch.
Does this cantilever significantly affect the structural design of the satellite, or is the payload adapter providing some support?
I know the whole thing is designed to withstand the vibrations induced by the launch but the structural requirements are somewhat different.