I'm just reviving this topic because it exists and didn't see the need to make a new one. Does Krikzz have something inherently against making an Everdrive for a CD based system?
The types of devices are completely different. Everdrives are all based on the same concept and prices, and adapted for each specific system and revision. he has been able to release so many and with different revisions because he has been able to reuse aspects of each one. Cd devices would mean starting from the beginning and need much more work. It's not surprise Krikzz has no interest in them.
Deunan's devices again are based on reusing the same concept, adapted for each system. Rhea was up and running quickly because most of the hardwork was done and just needed to be adapted for the specifics of the Saturn. His PC Engine device will again be based on the same work.
Cart systems are "easy" to do because they just copy the way the system was designed - the game is copied to memory and run from there, once the game is copied to that memory the system doesn't know any different from an original cart. The work is largely in creating code the system will understand to provide the menu and extra functions.
Cd's systems are MUCH harder because the systems are much more complicated as they rely not just on the cd drive, but various processors and memory systems in order to handle the read, process and understand the data. Some systems are also easier to do because of how they are designed. On Saturn and Dreamcast the cd drive is a module that can be removed and replaced with something else - GDEMU and Rhea are drop in replacements for the drive and so the rest of the system doesn't know any different. With some other systems such as PS1 the system is designed so that everything is on a single board and the drive assembly plugs into that without any of it's own processing. This means you can't simply replace the drive, and is the reason PSIO requires you to modify the motherboard and requires additional work.
It's for these reasons that Deunan's PC Engine product will only work on PCE+CD setups and not Duo's - the separate cd is much easier to modify than the single pcbs design of the Duo. Although even the version he has at the moment has issues - obtaining the necessary connector is the obvious problem.