Would I be correct in guessing the Retrode Game Boy adapter does not properly support MBC2 games: it can read the ROM just fine, but it does not detect the SRAM (because the SRAM is built into the mapper and so games will report no SRAM in the header). These are cart type 06 in the header.
Dumped about half my collection and I know these are MBC2 games:
Aretha I (though I know Aretha II is also affected)
Dragon Slayer Gaiden
Final Fantasy Adventure
Final Fantasy Legend 1
Kid Icarus of Myths and Monsters
Momotarou Densetsu Gaiden
Ninja Taro
Rolan's Curse 2
Tenchi wo Kurau
I know the MBC2 is set up to have 256 bytes of SRAM, but it is split using half a byte of A000-A1FF, and I'm not completely sure what Aretha does but I think it expects to read some garbage in the unused half of those bytes because as I recall it requires "RAM locking" to be turned on in bgb or the game will puke over itself when attempting to save. (though I suppose this will be harder to get support for, being that only a Japanese-exclusive RPG is affected)
Momotarou Collection 2 also won't dump correctly but I'm guessing the retrode doesn't like the few official multicart games as emulators don't seem to have good support for them either?
EDIT: Added a few more affected games.
I guess I'd need to have a few of those carts at hand to get it to run. You know, GB SRAM support is still at a rather early stage.