Well, it depends on how big the containers are and how cold your winters are. Being metal, the soil in them will get colder during the winter, so that might freeze your plants. If you have a wet winter, without drainage, the container might split because the water will freeze in the soil and cause it to expand and split the metal. I have two metal well buckets that I left out one year and they both split at the bottom. So far, they've not dropped the bottoms so I just left them as is for drainage.

Is there drainage? I have had various metal pots for years, but they all have good drainage. Guess it depends on what kind of metal, too. Mine are all galvanized, and heavy iron pots are popular. Seems like heat in summer would be more worrysome that cold in winter.