Before 0.92, it was very, very simple, and easy to understand and easy to use. Now, it is not.

• We have a setting called "Display units". As a user I'd expect this to only influence display (as the name suggests) but nothing else (i.e. I should be able to switch this at will without causing any change for my document)
• However, as soon as the user changes "display units" the "Scale" will change too! As a user I'm now frightened: Did I just change the size of my document? Should I revert to "Scale" = 1? (if they choose to do so it's very bad as then they'll mess up their drawing)
  
• The help of the "Scale" input is not helpful: "User units per {display unit}"
  As a user I ask myself: What is a "user unit"? If I (the user) just set "display units" to what I (the user) want to be displayed - is that a "user unit"?
  It obviously is not... A user unit is (even if it has a name that might suggest differently) something the user should not be bothered with (we don't have that term explained or used anywhere else in Inkscape I think).

While I'm no UI expert (who is ever? ;-) ) I think a possible solution to that would be:

• Rephrase "user units" to "{document,internal,SVG} units" or similar - something that makes clear that this is the unit stored in the resulting SVG document.
  (Problem: We used to call "Display units" "Document units" before 0.92, which was probably a bad choice and might cause ambiguity now)
  
• Potentially collapse the whole "Scale" section by default (not just the "Viewbox" part) - users should probably never change it anyway (unless they know what they're doing) and usually they also should not have to.
• Add a big warning sign: "Changing scaling will change the layout of your document - is this really what you want?"
  
• Rethink if "Scale" is a suitable term for what we are doing here. If I double the scale my document does not change it's size (but instead my content is scaled down). Maybe this is a language problem but my general impression is that in software "Scale" usually refers to "scaling (up) the whole image" (which we do not do) and not "changing the scale as on a map" (which is what we do).
  
• *Optionally* expose "user units" (with a proper alias) as a dropdown - if the user sets "display units" and "user units" to mm the scale will become "1".
  (Problem: How do we now what to put for "user units" when opening a file? For new files we could store it. For old files we could do the math and try to guess, but it might be hard to impossible in many cases).

Regards,
Eduard