It all depends on the material used. As you can find in our FAQ, various materials handle stress differently:
PLA, the most common 3D printing plastic used has good strength, but other materials such as PETG and Poly Carbonates handle stress better. Temperature is also a factor that needs to be considered when 3D printing. For example, PLA starts losing its integrity at around 50 degrees Celsius (this is called Glass Transition Temperature), while PETG can go up to 80 degrees. However, PETG has weaker UV resistance, which means it can become brittle under direct sunlight.