Preheat oven to 350 degrees F (175 degrees C). Grease 12 muffin cups or line with paper muffin liners.
Whisk flour, baking soda, baking powder, salt, cinnamon, and nutmeg together in a bowl; stir rhubarb and raisins into flour mixture.
Whisk buttermilk, brown sugar, melted butter, and egg together in a large bowl; stir flour mixture into buttermilk mixture just until all ingredients are moistened. Divide batter evenly into the prepared muffin cups.
Bake in the preheated oven until a toothpick inserted into the center comes out clean, about 25 minutes. Cool in the pans for a few minutes before removing from pan. Serve warm or at room temperature.
