Maybe there is some part that eMMCs have that SD cards don't, but SD cards absolutely do have a controller that does layout (wear levelling). It doesn't do the filesystem layer, but IIRC eMMC's don't either.

AFAIK the only reason SDs are less reliable, is because they're cheap and the socket can be loose, causing corruption during a write (or wear level). It would be interesting if you know of something logically different that also makes them less reliable.