1) The ROMs may only be set up for one region. Some Genesis carts still contained the Japanese text in it, accessible if you set your Genesis to Japanese (or use a Megadrive). If you put in Streets of Rage, you may end up with Bare Knuckle, or you may end up with "I don't know wtf to do with this so I do nothing." If there's no region lock on this machine (highly unlikely, I'd say), setting it for a specific region may be impossible.
2) As was said before, it's also possible that the GOAC can only read 512Kbyte ROMs. The chip on the board only needs to do what it's meant to do, nothing more. The reason the NOAC works out of those 10854929043 in 1 carts is that they also use NOACs for cart-accepting Famicom clones too, and it's just cheaper to keep making one type than various implementations for various uses. In short, assuming Sega isn't going to make a new Genesis using the same GOAC, they probably didn't go the whole hog.
And you can pretty much bet your bottom dollar that it won't read SMS carts. The Genesis 3 won't, and it has (as far as I've heard) a full Genesis cart slot. In fact, has anyone tried comparing this thing to the Genesis 3? That used a single-chip for most everything, right? There might be some similarities in implementation. (I know the Genesis 3's main chip was encased in more than a blob.)
For people who want to use it for non-portable reasons, keep in mind controller 2 probably will not be able to be used too
As far as the 68k and Z80 and everything else not being able to be put into that size of a blob, that is ridiculous. Look at an AMD AthlonXP. That thing has over 20 million transistors in that tiny rectangle on top. Which is like... 20 million more than a 68k and Z80 combined
I'm hoping that this is indeed functional as intended, but my hopes are a little lower than some. Still, may buy one anyway, because it is a neat product. Good luck to you all!