I've tried it at the past and "get it" but that doesn't mean i "agree with it" - to me immediate mode GUIs are the graphical equivalent to...

    Running = -1
    WHILE Running
      PRINT "1) Add record"
      IF HasRecords THEN
        PRINT "2) Delete record"
        PRINT "3) Edit record"
      END IF
      PRINT "H) Help"
      PRINT "X) Exit"
      INPUT "Choice: ", I$
      IF I$="1" THEN AddRecord
      IF (I$="2") AND HasRecords THEN DeleteRecord
      IF (I$="3") AND HasRecords THEN EditRecord
      IF I$="H" THEN ShowHelp
      IF I$="X" THEN Running = 0
    WEND

...which, sure, it is ridiculously simple (like imguis) but it can very quickly become unwieldy the more you ask from your user interface. There is a reason why UIs moved beyond that.

I think when you look at projects like Tracy, there's no question that immediate mode UIs also work well for non-trivial use cases:

https://github.com/wolfpld/tracy