In the bowl of a food processor fitted with a knife blade, process cream cheese until smooth. Add the mayonnaise, grated onion, garlic powder, salt, and pepper and pulse until combined. Add the shredded cheeses and pimentos and pulse to combine. Transfer the cheese mixture into a bowl and refrigerate for at least 30 minutes or up to 2 days. Place pecans on a baking sheet or in a medium bowl. Roll the cheese mixture into 1-inch balls. Roll the balls in the pecans to evenly coat them. Serve chilled or at room temperature.