Thanks for the question, because I'm sure it is a concern for many Mac music users. There is no definitive answer because it all depends on the features, functions, and ancillary software components you are using. We can certainly give you some details.

I personally have not updated my Mac to Yosemite (10.10.x), simply because even though the main driver and components I use are now compatible, I'm still in the middle of a few projects that I don't want to put 'at risk' at this particular moment. When it comes to computer operating system changes I always weigh the benefits and features added to my music-world by the new OS, versus the possibility of unfinished projects getting jammed up so how. But once I complete those Projects I usually go ahead and update. Since Yosemite does not have anything I would trade my current project for, I can afford to wait a awhile. If it contained music making related NEW must-have features, I'd be getting antsy, but since nothing improves my music-making, I can wait.

The YAMAHA STEINBERG USB DRIVER (there is only one driver for your MX) version 1.9.0-2 for Mac is Yosemite tested; an updated version was posted to download.yamaha.com on November 7, 2014. (I updated to 1.9.0-2 on November 7)

Cubase AI7 - Steinberg is still testing (it's ongoing) and has released a new installer, in case you would need to install AI7 on to a Yosemite Mac. Please see the information concerning Cubase and Steinberg products at the following link:
http://www.steinberg.net/index.php?id=content_and_accessories_saitfy&L=1

For all other software, plug-ins, and ancillary programs you might be running in the DAW, or standalone, you should, as always, check with the maker of that program. VycroMX, VycroMX Pro - you will want to check with PsiCraft Designs to see if there have been any known issues. I have not heard of any - but that is neither here nor there.

If you can hold off a short while... Usually around major trade shows things seem to catch up (or fall farther behind... ) NAMM is next week
🙂