I've used Slate products for a few years now. Back with X1 for sure (which I still record with because I prefer track layers over track lanes for editing. I need to convert, but I don't have time to fiddle around).
Anyways, I use Trigger, VCC/Mix Rack, Virtual Tape Machine, and FG-X.
I use Slate, UAD, and Waves a ton.
I don't recall mixing in X1 due to the passing of time and many projects, but X3 I have had issues.
I would open up fully-mixed session and most of the time VCC would throw a hissy fit and crash the session. I did submit the crash reports, but after months I gave up doing that. I was actually afraid to open mixes in front of clients for this reason. Nothing brings the vibe down like a project refusing to open 75% of the time (true fact). I eventually contacted Cake, who made a pretty good effort but ended up saying it wasn't their issue that they could see and to reinstall VCC.
Note that this was not the first time I contacted Cake about this issue. Just seems the last guy I got was willing to put more time into my issue and reply quicker ;)
This contact was around the time when Mix Rack came out, so I updated almost every Slate plugin to their newest version. I also updated to Sonar Platinum at the same time because I had a weekend without clients coming in and actually had time to do it and not rush.
So far.... no crashes when opening mixes in Platinum! Platinum crashes happen, often... but seem to not be Slate related.
My view on support:
UAD and Slate BOTH do not support Sonar. They will, and have, replied and offered to help. Slate was unable to help, but they did attempt to do so. Both are top-end plugins and I'd love it if Cakewalk would push their product on them a bit because we, as users, are left in the cold for choosing Cakewalk and working with high-end plugs from UAD/Slate.