Harmony is a product everybody says good things. The other devices from RetroHQ UK are nice they exist, but I would love dedicated Everdrive for those systems to expect something with the same quality and functions as in the existing Everdrives.
If you look for the idea of "there's already a flash card for that system, so no need for a Everdrive for it", so krikzz had no need to release a Mega Everdrive Pro, once the Mega SD existed before it... But he did the job, and between choose one of the two to get, I go to the Everdrive.
Firstly, those systems are not exactly trivial to program for, and they have tiny user bases compared to what Krikzz currently supplies for. He's free to tackle them, but it wouldn't make a ton of financial sense. For that matter, he ought to hit up the Virtual Boy. Though the DS/3DS I should think would be his next go since they have massive user bases. Lastly, the RetroHQ carts are really, really, really good. You won't be disappointed. Right now the owner has had to restructure his distribution but in a few months he'll have quantities widely available.
I think you jumped my last post, so I'll explain better...
The "FOUR competing devices" are just optical drive emulators, who askes to mod the system to install them, and compatible with specific models, except for the Satiator, who is a plug-and-play device compatible with any Sega Saturn model (and my personal choice). Even so, even if you can load disc images from those devices, some relevant Sega Saturn games askes for expansions from the cartridge slot, so any optical drive emulator will not be able to offer a complete experience alone.
Ask about special cartridges for the Sega Saturn for the Sega Saturn fans... There's a lot: The Backup cartridge, the 1MB RAM, the 4MB RAM, the KoF'95 ROM cartridge, the Ultraman ROM cartridge, and the Action Replay Plus are must have in any collection. Put in this pack other cartridges, like a modded boot cartridge to load the Pseudo Saturn Kai, or the Gamer's Cartridge. Now ask for any Sega Saturn owner if they don't like to get an all-in-one cartridge, with all the features from the cartridges cited, plus some extras. So, you can't tell it won't sell for sure... And I can't tell it will sell for sure. But my personal answer is: I'M WAITING FOR IT, even having all the essential cartridges in my collection, nothing can beat an all-in-one cartridge to keep it inserted in my console... And I'm not counting other things like backup the saves into the SD card, or simulate the Saturn Floppy Disc Drive. Who knows if it's possible to even run ROMs of Sega Titan Video into the Saturn (but I believe it's not possible to run ST-V ROMs into the Saturn, but who knows)...
The idea of an Everdrive for the Sega Saturn is to work as a multi-purpose device than a ISO loader (that was never the main point, specially because it was discursed it won't be viable to load ISOs from the cartridge slot before). As I said in my last post, using an Everdrive SS with the multi-purpose concept combined the Satiator will be the ultimate Duo for the Sega Saturn system: One load the games, and the other do the expansions switch. And it will be even better if the switch of the cartridge functions can also be set by software commands. By the way, there's a video where Professor Abrasive comment about flashing an Action Replay cartridge with a specific tool (like the Pseudo Saturn Kai) to work combined with the Satiator. So, an Everdrive SS with all features cited can be even better than a flashed AR cartridge with some functions.
And by the way, the main purpose of the Pseudo Saturn Kai is to enable non-modded Sega Saturn systems to play CD-R games in any region. Of course there's other functions, but lack others (extra RAM, etc...). There's the Gamer's Cartridge, who is a SD Card cartridge with the same purpose of the Psedo Saturn Kai, plus the use of the SD Card to load homebrews or to work as a regular backup cartridge (with not an 100% of compatibility).
I knew exactly what you meant, but I just don't think the development time and effort is going to yield much in return for Krikzz. I agree a cart like that would be good to have, but we don't even know if it's possible to do all of those functions with one device? I personally would like to get a RAM cart with game saving that didn't look like ass (Action Replay) ha ha ha. Just not sure Krikzz is the ideal candidate.