I had Harrison Mixbus several years ago when it first came out and honestly I don't remember my specific reasons - been awhile - but I do remember not liking using Ardour. Maybe it's better now.
I've been using Mixbus for several years now.
At first it was kind of off and on, but for the last two years, it has continually improved to the point that I don't use anything else.
The main thing that got me was the sound. It colors the sound pretty much the same as a Harrison Console would.
I took audio from Reaper and from Samplitude 10, and loaded it into Mixbus with no processing of any kind and was blown away.
Could you get the same sound with plug-ins? Probably, if you have the skill and the time, and the right plug-ins.
I render Real Tracks from Real Band and load them into Mixbus and they sound great.