I’m on El Cap now and have some graphic problems in cubase pro 9’ especially with slate digital VBC.
I’d like to know if Sierra is better before trying to upgrade my OS.
Thanks all!
It’s discouraged to use El Capitan with Cubase 9.
More info here: https://helpcenter.steinberg.de/hc/en-us/articles/207348390
El Capitan is substantially more stable with Cubase 9.0.1 on my system. I had one plugin blacklisted on El Capitain (and not Sierra) but the plugin devs (Relab) have already issued a fix.
Sierra is as rock solid for me as El Capitan was. I rarely have crashes with Sierra, even on huge sessions with immense amounts of plugins, and I had the same experience on El Capitan. The Cubase GUI issues, if you have them, are there regardless of which OS version you’re on.
Thank you all for the answers!
Waited until all drivers had been updated and updated than to Sierra. Working fine, not more or less problems than before.
Stable and besides that like Cubase 9.
Just a little update on my part, I now can reproduce crashes with u-he plugins. I’ve upgraded to Sierra again and will be giving this a more thorough test by isolating various plugins.
I’m currently in touch with the u-he guys to determine the cause of the crash with their plugs 
After further testing, I’ve been able to crash Cubase 9 on Sierra with NI, Softube and Soundtoys plugins so far, simply when creating the plugin. The crashes are random and not consistent. I’m currently trying to work with the various vendors to see what’s up. Currently in discussion with Soundtoys to start with.