EF3 and EF1 are very different things. For example the EF3 has a boot menu, kernal replacement, AR 5, SS5 emulation and 8 slots. It also has a usb port that lets you flash from PC, download and run a prg from the pc or write a disk from the PC.
It will "work" on a 128, well I don't think Kernal replacement works, but when you exit the 128 mode the cart is 95% disabled. You get 256 bytes of RAM at DF00 and that is it.
When making games in the emulator it is important to test on hardware quick and fast. Balancing for an xbox controller on a pc is a bad idea. You put it on hardware and its bad. I need a fast way to get and test on real hardware. There is no option available for this. RR-NET is 64 mode only, EF3 is 64 mode only. But for testing I make use of Function ROMS to add debugging on the 80 col monitor. So I want to get a fully working EF cart, that lets me download and run a file from USB and lets me flash External Function ROMs ( banked ones ). I also want to get banked RAM ( the EF3 actually has 32K ) so I can set up a RAM external ROM, that you can program 256 bytes at DF00 and then let the ROM be accessed as EFUNC. This way I can USB download my debug function ROM, multiple banks of PRG from the PC and test on hardware.
I made a custom solution where I have a userport transfer program in the internal function ROM to boot it quickly on reset and then a Raspberry Pi connected to it to get network connectivity. It makes it possible to send programs to and from memory and disks. However, I have no solution to test ROM images for cartridges. This could be a way to do that perhaps.
I'm also working on a Raspberry Pi based solution a PiDS but getting parts is hard at the moment. Even when all the parts said "I'm in stock ready to ship" somehow one of them is on back order... But having the EF128 cart give me a RAM function ROM will also help me get my full testing debug setup working as well.