Unfortunately Retro-Bit receivers only work with their own specifically paired, officially licensed controllers. Because they utilize proprietary 2.4GHz or non-standard Bluetooth signals, they cannot connect to other third-party controllers or modern console gamepads unless the makers of other third-party controllers programmed them to work with Retro-Bit receivers (which for now they didn't).
True, but say you wanted to play any of Dreamcast' fighting games. If Retro-Bit released a receiver for Dreamcast, then you can use a wireless Sega Saturn controller for it.
Now, granted, the D6 wireless controller costs $50. But, the
standard wireless Saturn controller costs $35 and comes with free receivers for all Retro-Bit controllers!
While, yes, the Pro Controller I got was also $50, I'd say it's still better than the D6 for having XInput in mind. But it also came with two receivers, one for the Sega Saturn and the other for my PC.
I don't know the price of upcoming Retro-Bit Dreamcast controllers, but if any of them also include receivers for both PC and Dreamcast, that means that while it would cost... I'm guessing, $70-80 for a wireless Sega Saturn controller and a wireless Dreamcast controller, it's wireless receivers for respected consoles will allow you to use of the two controllers for any of the two consoles. But what I'm point out is, while it still costs more than the D6 to buy two controllers, you're still technically paying less than the D6 to have a wireless Sega Saturn controller for the Dreamcast.
Or, if available, you can buy a wireless Bluetooth receiver for $15 for Dreamcast if you don't want two sets of controllers. 35+15=50... so I guess roughly same price as the D6, but the receiver at least has multiple uses.