I haven't used big rocks as anchors for winching but I can tell you from my climbing experience that even very large, car sized, rocks can be unstable. I have personally accidentally moved rocks that probably weighed over a thousand pounds. On the other hand, I've rapelled off of or anchored to hundreds of big rocks or rock protrusions without incident. Granted the magnitude of the forces involved with body weight verses a loaded rig are much different the concept is the same.
Before using a boulder or rock protrusion as an anchor give it a careful inspection, look to see if it is well into the ground rather than sitting loose. Try pushing and prying on the rock to see if it moves. With big rocks this might require several people or a large pry bar and trying to move it from various angles or climbing on to it to see if it will shift. You might even consider giving it a test pull if possible.
If you do find what looks like a possible anchor be sure to use a sufficiently long sling to allow a shallow angle for the anchor and protect it well from abrasion. A tree saver or non-stretch type strap is ideal and chain or cable works well too but be careful that they are not kinked or forced to bend around sharp edges. If equipment allows, one trick is to completely wrap the anchor with the sling/chain etc. to keep it from shifting if the direction of the load changes. This system would partly depend on the type of strap/chain and shackles you use.
