For storage in particular, iBoff made an adapter for NAND chips that places them on a carrier board: https://www.youtube.com/watch?v=E3N-z-Y8cuw

The whole setup (allegedly) fits inside original chassis, too, and disk speeds are about the same. So the only real tradeoffs for Apple are cost and the fact that user can swap in third party parts instead of paying obscene prices Apple charges for spec upgrades.