Undo not working in blender 2.8 According to: 

my undo doesn&#39;t work in 2.8, except in edit mode

Global undo should be enabled in order to undo operations done in the editing tab. In my case, I added faces between two edge loops created with Ctrl + R; I deleted one (it had a face with odd number of vertexes) and created one above it. Everything looked ok; faces just needing to be added in between the edges. I select both loops (Alt + click) and hit F; the result was a wavy surface that now I cannot delete by hitting Ctrl + Z. The wavy surface appears only if the face is created from the loops and not from the vertexes individually.
I was impatient; blender is pretty slow on my machine. Just close and restart every now and then.