Is the game support likely to be different on these two devices.. I appreciate that both do not support NG: Dev games, which is a good thing, but can both run any unlicensed or new homebrew game or perhaps a specific hacked ROM. Or is one fixed to run only a specific ROM list..?
[PURE_SPECULATION]
Because there are multiple banking schemes and protection schemes (potentially incompatible and conflicting between games), the FPGA needs to know which cartridge hardware to emulate when you load a game. There would have to be, in effect, a 'database' of supported games stored - presumably in the ARM - that recognises which game you've selected and activates the appropriate logic in the FPGA, or possibly even loads a complete new FPGA image. Not too dissimilar to the various drivers that comprise MAME's Neo Geo support. So support would, I believe, be limited to the "known & supported" commercial titles at this time. This is (also) how NGDEV titles remain unsupported and all-but-impossible to hack by a 3rd party to add support.
Note that it should be simple for
neosd to release new firmware in the future to add support for new games should that become necessary, likely via SD card.
Now because not
every game's cartridge hardware is unique in this respect, it is likely that one (FPGA) configuration is able to support multiple games. The simplest example is no custom banking and no protection which should also allow (most?) homebrew games to be supported. So either the firmware defaults to the simplest configuration for unknown games, or the homebrew developer needs to masquerade as another, compatible, title as one does when developing under MAME for example. The latter case is by no means convenient, but at least gets you up-and-running and I don't see any reason why
neosd wouldn't add 'proper' support for homebrew titles if there isn't any already.
[/PURE_SPECULATION]