On my Solidworks 2017 SP3 installation, changes made to variables using the Equation Editor dialog are not added to the undo buffer. To reproduce:
1) Open or create a part with geometry tied to global variables and equations.
2) Open the Equation Editor (in case it matters, I added mine to the toolbar with a "Σ" icon and access it from there)
3) Change at least one of the variables and press OK.
3) Observe changes to your model geometry (Rebuild if needed).
4) Press CTRL+Z to undo. The last edit is not undone. If you did previous model edits, the last change you made before using the Equation Editor is undone instead.
Can someone please verify this behavior on a more recent version of the software?
In my opinion any changes you made while the Equation Editor dialog is open should be treated as one operation and appended to the Undo buffer when you hit OK to the dialog.
While I'm at it, the repaint performance of that dialog is terrible, even when "Automatic Solve Order" is unchecked. If you have more than about 50 entries in Ordered View (which is not unusual considering it lists all your dimensions) you start to see slowdown when opening up the dialog or on each keypress while typing a filter. Seems to get worse as you add more entries (dimensions / variables / equations).