Yes, nightly builds are stable if you use a released version. Actualy, they are nightly build of the latest git version of the concerned branch. They are an easier and automated way to update your Odoo installation than git clone.

But, if you need bug fixes, you may prefer to git clone the OCB fork of Odoo, because it contains fixes backported from upper versions. See Odoo Community Backports https://github.com/OCA/OCB

I suggest cloning Odoo directly from https://github.com/odoo/odoo and checking out to the "13.0" branch. You will always be on the latest release if you regularly do a git pull and restart the Odoo service.