The only way I have been able to get MOTU's MachFive product to work in Metro is to remove all traces of the VST plug-in from the folder /Library/Audio/Plug-Ins/VST/
That is remove the MachFive folder. In my plug-ins folder I simply created a disabled folder and dragged the MachFive folder from the /Library/Audio/Plug-Ins/VST/ folder to the /Library/Audio/Plug-Ins/Disabled folder.
The important thing is that Metro does not find any MachFive VST plug-ins. Once this is done, the AudioUnit (AU) version of MachFive works fine.
Why this is the case? Only MOTU can answer that.