To properly appreciate this, you need to understand that the cold hardiness of your  outdoor bonsai trees will not be equal to what they can withstand as full grown trees with their roots deeply buried – read protected – in the ground, where they cannot freeze.  In a small shallow pot, the roots can easily be frozen if they’re not well protected and this is where the beginner needs to take the most care.  Happily once you protect the roots your Juniper is not likely to experience die back in the branches when the temperature dips below freezing.