My only gripe with Mixbus is how UAD/Plugin Alliance VSTs will not save settings between sessions if you "start" with a preset (and is still true if you tweak said preset). Making a custom preset has the same results.
Here's the kicker (at least with UAD, not sure this is true for Plugin Alliance). If you reopen the session, the preset name is still there (with all zeroed values). If you re-select the preset of course the values will return. Added bonus, if you did tweak the preset originally, after re-selecting the preset name the tweaked values return.
Seem like a pretty simple bug to fix because obviously the session/project knows what the values are.
The flip side good news is if you just insert those plugs and tweak from the default (i.e. don't start with a preset) the settings will retain between sessions. Go figure.