It’s worth noting that u and ctrl-r don’t operate on an undo stack but on an undo tree.
So if you have a series of edits and undo them all then do something else you can still get back to where you were before.
Use g+ and g- to walk the tree and the Gundo plugin is gives you a nice visual diff interface.
So having a separate shortcut and “stack” is actually a great feature of Vim IMHO.