the really and only advantage of the SGFX was the real dual layer scrolling, which had to have be done on the PCE either via h-sync scrolling but which only allowes to scroll in different speed on horizontal scanlines (no object overlapping), therefore it isn't a real parallax scrolling of its definition, or via used sprites (huge sprites). good programmer could use a lot of above tricks to simulate excellent multilayer scrolling (winds of thunder, dracula x, magical chase etc.), but which also had sometimes some sprite flickers as a result.
but for many other not such experienced team it was kinda shuttered or they where to lazy to use the advatage of such tricks.
that why almost any cheap MD games came with parallax scrolling, while the pce don't.