It all depends upon your climate and materials.
Clay bake ovens exposed to rain don't last long. The outside just doesn't heat/fire hot enough to seal it from rain. Some people have added an outside "stucco" type layer of clay mixed with cement/concrete. That layer does a lot to repell moisture, but needs constant touch-up/repair of all cracks to maintain that seal. But the constant heating/cooling causes expansion/contraction of the oven - which leads to cracks, especially in any HARD outer layer.
Most people tend to build a roof over their bake oven. This keeps the rain off. This can be as simple as a tent, a shed roof, or a more substantial structure. They also build up the foundation under it - out of stone or concrete/cement. This keeps the foundation dry and deters shifting. Just like a house, keep it's head and feet dry. So, a HAT above, and a solid footing that won't shift in the weather underneath.
Any clay bake oven will need periodic care/repair. It's just how they are. But the better you protect it from the weather, the less repair will be needed.
Just my humble thoughts to share.
Mike Ameling
p.s. You just won't believe how good bread baked in a clay bake oven tastes - hot out of the oven - with fresh butter!!!!!!