Steam mops essentially push water through your floors. If the floors are not sealed or if the sealant is wearing off, it will not be able to prevent the steam from pushing through your wood floors where it will eventually cool and form water droplets in the wood. Excessive water in wood causes it to buckle. This makes the floor boards separate from one another and can cause them to creak. This is not likely to occur if you use the steam mop every few weeks or months, but it could readily occur if you use the mop every day.
When water seeps into the wood it can cause wood rot to develop. This essentially softens the wood to a point where it can no longer handle wear and tear. While buckling can be fixed with great effort, wood rot generally means you'll have to replace your wood flooring. Again, this will generally only occur if you use a steam mop every day or if you don't use it properly.
Water from the steam cleaner can get in between the layers of finish and stain and cause them to begin to pull away from the wood flooring. The heat can also cause damage. Heat issues will manifest themselves in peeling or bubbling areas on your flooring. This occurs when you don't use the machine properly and if you use it too often.
Avoid damage by only using the steam mop to deep clean your hardwood floors every month or every few weeks. Always cover the bottom of the steam mop with the microfiber pad that is usually required for hardwood floors. This creates a buffer so the floors don't get very wet. Never depress the trigger to release steam while you are not moving the steam mop. Pressing steam into only one area is a quick and easy way to ensure eventual damage.