Wait.. Kpj told me a MVS bios wouldn't work in an aes? But it does work the other way around? I was trying to figure out the issues XX and I were having with the neogeocd unibios.
MVS BIOS on AES console doesn't work because the MVS expects there to be a backup circuit, which the AES lacks. The MVS BIOS will display a Backup RAM error every time.
AES BIOS on MVS board works because, among other things, the AES doesn't care about the backup circuit, so it works fine. Of course, this means you won't have high score saving, bookkeeping, soft dip settings or possibly hard dips either, also no coins. But hey, games will still work.