Rocks come in different shapes and sizes. Big rocks should be cleared if possible. If not becareful working close so not to damage the head. Medium sized rocks don't fly up as they are too heavy but could damage the head if sweeping low.
Only small rocks that fly up which can only harm you if hits the eye. That is why you wear safety glasses.
In Thailand I would check for broken glass first and clear it if there. Lots of drunks chuck out bottles.
Otherwise the main factors are a powerful machine, good spinning head, good cord and good technique. Weak machines struggle in tall grass. 35cc breezes through compared to 25cc.