It is not common for guests to post complaints unless something really upset them (vrwd.org received only about 50 vacation rental complaints last year, but I'm sure there were many more unhappy vacation renters than that!). I am not sure what the vacation rental complaint statistics are... however for restaurants for example, only 1 every 20 unhappy customers files a complaint.
In any case, usually it is highly recommended that home owners do everything possible to please an upset guest. An unhappy customer can ruin your vacation rental trust and affect your bookings in a negative way by posting their complaints online.