I assume subject says it all but will the MEGA65 be able to support this cartridge? 
I am not exactly sure, if I even will need the functionality from this cartridge, as I assume that I will be able to browse for a disk image or a program from its SD card directly via the MEGA65 UI somehow and I hope it will have some fast-loader built-in?
If it will not work, what is then the reason for it and will it ever be able to work (future maybe)?
Don't know about the Ultimate-II specifically, but:
- the MEGA65 already includes a Freezer Menu which lets you select the disc image. Currently .d81 images are supported (as the C65 had a built-in 3.5" drive), but 1541 support is also planned.
- the 1565 (==internal drive of the C65) was already many times faster than a stock 1541. The MEGA65 is even faster when accessing disc images. While not as fast as the SD card interface could be, it's on par with current fast loaders.
As for cartridges: Some work, some (currently) don't, but it's constantly being worked on. But I think once the MEGA65 is finished, there will be no need for the Ultimate-II cartridge.
However the MEGA65 is not having USB connectors, and the Ultimate-II does. It would be versatile to have U-II+ support.
The last time I tried, the 1541 emulation part of the Ultimate-II+ was working, but I was unable to get into the cartridge menu. AFAIK the DMA cartridges are not supported yet, so there is no chance for it to fully work (REU…), besides the MEGA65 IEC support is currently not too compatible (for example, JiffyDOS won’t work properly yet).
ubik Ultimate-II+ is much more than just 1541 emulation. It can emulate 2 drives, they can be nicely expanded (you can use S-JiffyDOS ROMs), it simulates the Datassette, REU, geoRAM, some cartridges, it even simulates IEC printer. I can see Gideon is experimenting with 1571 and 1581 emulation too ( https://github.com/GideonZ/1541ultimate/commits/1581 ).
There is no chance for MEGA65 to provide all this in the near future. True, we can handle D81 images, but the internal drive is not compatible with 1581. And 1541 is not there yet.
