On the table in the article linked below, CC121 shows “no”, as in “not compatible” with Win 8.1.
No note is given as to whether any update is coming. Huh? 
Is this OFFICIALLY the end of the line for CC121?
Happily, I do seem to be running it just fine under Win 8.1 (upgrade)…
THANKS
It’s the same for the CMC series stuff as well, I started a thread in that section asking about it but no one from SB has bothered to get back to me.
I’d really like to know if both lines are expecting upgrades to be compatible with 8.1, seems completely stupid that they’re not, CMC is still the current line of controller and the CC121 is not exactly that old and really does deserve a swift update, 8.1 has been out a few months now.
Is 8.1 really that different to 8 or is just a case of it hasn’t been fully tested?
Wow really no support?
A told a friend to buy it right now because it would be great for his type of workflow.
I will tell him to wait for the puchase until this is solved!
This is not Apple so you don’t have to throw away your existing hardware or software just because the OS got an update. Windows software and drivers are usually forward compatible so even devices with Win2000 drivers should work with Windows8.1 (with some exceptions).
The article just says that you can not force Steinberg to support you if you have problems with the CC121 and Windows 8.1. But this is just a lawyer thing and I doubt that they’ll leave their customers alone with if there are problems anytime soon.
This also means that the device is now officially deprecated. We’ll have to see if the CC121 is still supported by Cubase 8. Maybe someone from Steinberg can say something about this?
I am currently looking at the CC121. If it’s not compatible with win 8.1, it is better to wait, I think.
Does it work with win8?
Regardless of whether the CC121 or the CMC series functions fully with win 8.1 (and so far I’ve not heard either way) its really disappointing if SB have indeed dropped support for this hardware. Both are still very much in use by lots of people I know and the CMC series is still the current ‘made for cubase’ controller, dropping support so quickly makes me very unwilling to invest in SB hardware again.
Internally the CC121 is a simple Yamaha MIDI device that uses the same driver as all other Yamaha MIDI devices.It works even out of the box under Linux. So I doubt that it will stop working anytime soon from an operating system perspective.
My biggest concern is that the CC121 will not be supported by future Cubase/Nuendo/Wavelab versions. These applications require dedicated add-ons to support the device and the APIs are not open to the public. This means that only Steinberg can keep the add-ons up to date. If they don’t do it then the device turns into an expensive doorstopper unless you refuse to update Cubase or switch to Reaper.
Updated to win 8.1 64 bit, and yes at first the CC121 and my mr816x didn’t load/function properly, after reinstalling the latest drivres they worked again. No extensive testing done just basic operations.