I use a mixpre-6 (non-M), with the musician plugin installed. The M and the non-M units run entirely different software. It’s not really accurate to call the “Musician Plugin” a plugin. It is a standalone app that doesn’t relate with the standard Audio App. They have very different workflows.
The musician app is meant for multi-take/multitrack recording. It stacks tracks like one sees in a DAW. Each track is an independent WAV bundled in a “song” project. All the tracks share a single timeline. For me this is very much not what I want while field recording when I’m recording multiple sessions with multiple mic placements. There are workarounds: mute tracks, have a very long track, unique project for each mic placement, etc, but they are all a big PIA.
The Audio app is not meant for multitrack recording. It does not stack tracks/takes. Different takes/tracks each have their own timeline. With the Audio app each recording session creates an independent composite WAV file. These are bundled in a project folder. This app also alows for much more control over the recording. It has Advanced and Custom control modes that allows for the assignment of knobs, gain scale, etc. The Audio app also interfaces with Sound Devices Bluetooth app allowing remote access to a level monitor, meta data editing and remote transport control. This is very handy for reducing handling noise.
Personally, I was frustrated using the Musician plugin for field recording. However, I find it great for recording modular music.