Adding to this idea:
https://illustrator.uservoice.com/forums/333657-illustrator-feature-requests/suggestions/32403850-make-units-and-grid-settings-to-be-bound-to-file#comments

I work with both printed and digital formats, on top of that much of my work is multilingual, involving English, Japanese, Portuguese and other languages (at least we don't deal with LTR type... for the time!).

As such, I'm constantly changing units from mm to pixels and points, type from points to mm to Q, it's a mess! And I do that several times a day.

I guess the easier path would be to bind the units settings to the workspaces, as binding to the files would require changing the .ai format. But what do I know?... Please make this a feature already, it would give me a reason to actually use Workspaces for a practical purpose besides just backing up the position of my palettes...

If Adobe decides to assign the units to the workspaces:

1-Please also consider adding a visual cue to the units somewhere in an unused part of the UI (i.e.: besides the tool indicator in the bottom left of the document screen? Something like [Unit: px Line: mm Type: pt EAL: Q]

2-Please don't stop at units and assign also the units for increments of text and leading, tracking ems, baseline shift and nudging, also drawing angle and rounded corner radius, and grid spacing and grid subdivisions! (hehe)

And last, if you do this, please tell the Photoshop and InDesign teams to do the same.