Hi there
Guy. I am a Harrison Mixbus fan too, I love it. One thing I found about plugins and especially getting Mixbus to scan them is I found it is much better to create a folder and only put copies of the DLL's in there you want Mixbus to find. Then point Mixbus only into that folder when scanning. It will take far less time and you will only have a short list of the ones you really want to use inside Mixbus.
If you don't do this it will scan your whole system which not only takes a long time but I found plug-ins that Mixbus did not like.
It does depend on you either using Mixbus right from the start to create everything within it and mix there too. Then you will want a lot more of your plugins available to it.
I use my DAW to do everything right up to the mix stage. I export tracks and buses for importing into Mixbus. I even export the reverb returns and put those stems on a track as if the reverb were inside Mixbus. Although I did get their reverb plugin and it sounds very very good too. Export any special plugin effects and any EQ's that are either surgical or specialised. Leave any broad based EQ and dynamics off when exporting and do that in Mixbus. It handles both of those things really well. It sounds fabulous too.
It is excellent even for just mastering. The mastering EQ and multiband comp are also very nice too. I also like the Sonitus stuff a lot and I asume you got that working OK as well.