Okay so I got to test out the Retro Freak this morning. All the cart slots work good. This isn't a full review, just some notes.
For NES carts I used a Tototek Famicom adapter. The adapter works great in the Famicom slot, and like the Retron5, if you remove the game from the adapter while the system is running, you can continue playing. Removing the now empty pin adapter from the REtro Freak results in an error.
My Mr Gimmick repro with expansion audio was recognized and boots in the Retro Freak. No surprises here; it is an expansion audio reproduction from Flashback Entertainment utilizing the INL FME-7 PCB with onboard audio. To the Retro Freak, it is probably identical to an authentic Japanese Gimmick.
Game runs fine, although there is a slight emulation error where certain notes will hang for a second or two when Gimmick dies. This does not occur on real hardware.
Also recognized and working is my NES NTSC Earthbound proto. These are fairly common reproductions so it's likely the ROM was added to the database. None of my other NES homebrews or repros that I have tried worked. RetroUSB's NWC is a no go. No surprises there.
Genesis/SNES/GB slots all work with homebrew games:
Genesis: I have tested Fix It Felix Jr (Genesis) homebrew created by NA member Dra600n of now defunt Airwalk Studios. The game was unrecognized, but I run it anyway and it worked great. Sonic 2xL hack. A Gag ROM hack of Sonic the Hedgehog 2 where the golden rings are replaced by "Onion Rings" and by eating too many of them Sonic gets fat. The game was unrecognized, but I run it anyway and it worked great.
Sonic 2 + Knuckles lock on recognized and worked great, so lock on tech seems to work good.
SNES: I have tested some Piko Interactive homebrew. Classic Kong was unrecognized but works great. Noah's Ark 3D does not work at all. I do own but haven't tested the Piko 4-in-1 yet. This game was supposed to be made to work on the Retron5 so I'll need to test it on the Retro Freak. There's a strong likelyhood both systems are using the same software.
Game Boy and PCe. I didn't take a lot of time testing these. Shrek 1 and Shrek 2 full movie GBA video carts definitely do not work. Some massively large GBA video carts (full movies) used bank-switching to allow greater than 32 megabytes of data on a single cart. Any GBA cart that uses this bank-switching will not work. No real loss there. AV quality on these titles sucks donkey and HD will only make it worse. I did ren one Game Boy Color homebrew, Chunkout. The game was unrecognized, but I run it anyway and it worked great. I only tested one game in the PCe slot, US TurboGrafx Devil's Crush. Recognized and works great.
To do list: test my one Turbografx homebrew: Atlantean by Aetherbyte. Test the Turbo Everdrive v1.x without memory card inserted to see if the Retro Freak can successfully dump the flash ROM. Redundant since you can backdoor load ROMs, but cool nonetheless.
In summary, most vintage game carts that use standard mappers should play fine. Any NES hacks, homebrew, or repros are not likely to work unless they are well known and in the public ROM databases (Mr Gimmick or Earthbound NES protos for instance). Any homebrews utilizing custom flash mappers like RETROUSB or MEMBLERs will absolutely NOT work and probably never will.
Homebrews, hacks, or repros for other systems are likely to work as long as they don't do anything weird with the hardware. Flash carts are redundant because unlike Retron5, Retro Freak supports dumping as well as backdoor loading of ROMs through the Micro SD card.
In other thoughts, I let the Genesis emulator run for a while with attract mode and filters turned on, and the Retro Freak rebooted. Upon closer inspection, the brain was very hot. The module docks into a slot in the bottom. Docking blocks ventilation when plugged into the cartridge dock, so console may have overheating issues.
Lag: Currently I get slightly less lag with the Retro Freak hooked up to my 2006 Sanyo LCD than I do with original consoles through RF or composite connections. Further testing will need to be done on my low latency (~9ms) 1080p ASUS monitor before I can make an honest judgement.
Pros: English menu option. Far superior build quality to Retron5. Dumps ROMs to SD. Allows backdoor loading of ROMs via SD. Brain unit can be undocked from the cart loader for portability.
Cons: Built in Manual is Japanese only. No wireless controller option. Needs pin adapter to play NES, SMS, and Mark III games. Custom NES/FC cartridges (hacks, homebrews), pirates, or vintage games with highly obscure mappers not likely to work. May overheat when plugged into cartridge dock. Controller multitap costs extra $$$.
Other thoughs: RetroUSB will be releasing the AVS (HD FPGA NES/FC console) sometime early in 2016. This will purportedly support all NES games, peripherals and accessories (sans Zapper) with 100% compatibility. The Retro Freak pretty much covers all the other popular systems, sans N64 and Atari. If you absolutely need NES homebrew and repro support with HD output, either the RetroUSB AVS or a Game Tech modded HDMI NES is currently the only way to fit those needs.