It complements the existing issue tracker, the issues and the labels are the same. You can use issues without the board but it might be useful to give a visual overview. In meetings it is frequently easier to use a board see the many issue at a glance and to drag and drop them.

I'm not sure we want to have the software enforce limits, but I do agree that you don't want any list/column to become too long.

Cycle time analytics are planned in https://gitlab.com/gitlab-org/gitlab-ce/issues/20975