Wind: How sheltered is the 3D sign from wind? How long do you want it to last? Depending on these answers you may wish to use 3D shapes that better defend against wind. In nature, the sphere is the strongest 3D shape by far. In construction, the triangle is often pointed to as the most enduring shape. Thus, in situations where wind is a serious environmental pressure on a sign, carved 3D features that are based around these two shapes are often the best choice to aid with overall durability. Also, consider the angle of the wind. It is wise to avoid placing shapes with large linear planes (such as cubes) in such a manner on a sign as they face the wind head on, because this ensures the greatest amount of force along the weakest section of the shape. Similar principles should be applied when designing the structural 3D supports used to attach 3D shapes to a billboard.
Temperature: Temperature, and changing temperature, is another major consideration for 3D signage. Choosing the right protective coating for 3D components is of paramount importance in order to prevent the finish from cracking and peeling because of hot or cold temperatures (or the transition between them). Is the sign permanent or seasonal? What is the local climate like? What level of protection will you need? Protective coatings are also important because they can contribute to wind resistance, both by enhancing the strength of the 3D carving and by protecting the carving from debris carried by the wind.
Moisture: 3D elements on a sign can also serve to encourage or inhibit environmental damage caused by moisture. How exposed will the sign be to precipitation? What is the humidity? How realistic is care and maintenance? Shapes that form natural pools and indents will collect and hold water (a potentially damaging trait) while shapes that curve downwards will reflect water and encourage it to drain away. Likewise, shapes with significant flat vertical feature can accumulate snow and ice which can cause damage or obscure large sections of a sign. Knowing and accommodating for these contingencies can make a huge impact on the longevity and usefulness of the sign.
A few moments of reflection during the planning and 3D modeling stage of 3D sign design can therefore prevent major problems down the road! Shape, orientation, coating, and angle can all significantly help or harm the durability and longevity of sign. Food for thought!