Regarding the clones, Qtech and Takis. There's little nuances appearing, they're not complete clones.
Eg the latest Next core, uses the main sd to boot, and it's the only core that can use the secondary sd as another drive, well that didn't work on the clones, as according to Sorg they don't have that bit wired up, so the core had to be modified for that to work.
Other ones I've heard of, no sound in various arcade games, again due to a shortcut. Various gfx problems, various input problems. I'm sure more things will materialize.
Don't get me wrong, I bought Takis megapack as a 2nd Mister for 150quid, but it's not a complete clone, as I found out and made it dual ram. The dual ram works, but then you can't plug in the audio board, so don't get analog sound out, only digital. Nor do you get it via HDMI, cos that bit isn't there either. Bought a 20quid convertor this week, to convert the fibre 3.55mm lead digital sound back to analog. The public are buying them in 1000's, but the cores have to be modified in the case of the Next to work with them, most of the devs don't like them. It's hard enough to copy a chip with 50,000 transistors on it without having to fanny on with other stuff.
I wish I knew Verilog, and had the patience. The devs are fantastic people, I pay most of them on Patreon. Sorg for everything, Jotego, Electron Ash (do you read his posts on the Dreamcast core in discord, it's fascinating stuff). Even if it is aimed at Mikes Replay 2 which should see the light next year.