The reflog is very helpful but I don't think it counts as an "undo". Some operations (like git add or git push) won't show up in the reflog.

Even the operations which do show will often require more thought to undo than a hypothetical "git undo" would. I know how to use the reflog but I often go out of my way to avoid it because "git branch tmp HEAD; git $POSSIBLE_MISTAKE; git reset ---hard tmp; git branch -D tmp" requires less effort than deciphering the reflog's output.