I tested in Cubase 9 Pro today, macOS Sierra with my Mac Pro trashcan. It's a clean install since I upgraded to Sierra last week. Some notes after a one-hour session on a somewhat light project (2 instances of Kontakt, 1 of Massive, 2 instances of Stutter Edit - about 9 tracks total loaded with Neutron):
Track Assistant managed to do a few things I hadn't considered with a few tracks. On the whole it guessed bass and percussion great. It thought some of the Stutter Edit stuff I was doing was guitar, so close enough. Piano on the mid-high register with a lot of reverb it kind of shrugged on but cut the low end and raised the highs. The EQ curves it assigned were not 100% spot on but with a dense project I could see an argument for it if you're pressed for time or want to learn WHY it's making those choices in those situations. A good learning tool!
I am my own worst enemy when it comes to the masking feature and if you plan on using Neutron heed this advice: Keep everything clearly labeled.
This is beginner stuff but I'm lazy, so this older project I was using had tracks labeled "Audio 01" and "MIDI 03". Not a big deal for me but when you start loading instances of Neutron into a project, it names them "Neutron 1" etc depending on which tracks you put Neutron on first. So when I put Neutron on my bass on MIDI 01 and then on my pad on Audio 06, it named the instances within Neutron "Neutron 1" and "Neutron 2" when I went into the masking part. You can rename each instance of Neutron on the top left corner of the plugin window and the instance will take that name globally. I'd strongly recommend naming your tracks, then making sure your Neutron instances get named exactly the same to spare confusion.
As for the actual masking function, it's great. Just make sure you're thorough about checking everything. You can only A/B with Neutron, so only two sources at a time - at least that's what it looked like to me while using it earlier today. So a kick and a bass you can check for sure, but a kick and a bass and a low tom you should def use your ears to make sure things don't sound funny. It was easy with two sources to see where the muddiness was cropping up in my project.
As I flesh out the track I was using with it more, I'll have more input for sure. I did a test export of the track so far so that I can A/B that with what I would consider the finished project.
Also I forgot to mention - and this could be Sierra, Cubase 9 Pro, my setup or Neutron itself - I noticed my CPU started getting loaded with about 9 instances of this. I was hearing some crackling too but it could have been that I haven't set my latency yet in the app. I wasn't expecting it to go easy on the processing but the reviews I've read all say that it was a light load. I have one of those crazy workhorse Mac Pros w/64GB of RAM and the processing was up around 60-70% with what I'd consider a light project (~10 tracks with some FX).