I have never heard of SoCs and I don't know anything about Raspberry Pi, although I understand it is a lot more powerful than a Mega. The thing about the Mega is the memory. On a Uno for this sort of thing, you can run out of memory before you run out of pins. In your case, it looks like you will need the pins as well. I have no idea how you would drive 64 relays, but I bet it can be done, particularly if you use mosfet or the like.