Tried making banana bread tonight with a brand new mixer and my bread just burned and stayed watery. I’ve used this recipe before. Was it the mixer?

Wow, that sounds really odd. If it didn't rise, it makes me wonder if there was something not quite right about your chemical leavener (baking powder or soda). Also, the mixer may have liquified the banana too much.

I've never found that it made any sense to use a stand mixer for banana bread, simply because it doesn't need to be beaten that much - unlike many cakes, which require air beaten in, for lift.

My standard banana bread, adapted from the Tassajara Bread Book: Heat oven to 375 degrees.
Prepare a large loaf pan or 3 minis + 3 muffins, or 4 minis. Whisk together in a medium bowl 258 grams all-purpose flour (or part WW and part AP) + 1 teaspoon baking soda, 1/2 teaspoon baking powder, pinch of salt and whatever spices appeal to me (sometimes cardamom + nutmeg; recently, pumpkin pie spice). In a large bowl, whisk 90 grams of brown sugar with 90 grams of cooking oil; beat in 2 eggs plus a splash of vanilla extract. Mash ripe bananas to measure a bit shy of 2 cups (four medium); top off with a splash of buttermilk. Beat 1/2 of the bananas into the wet ingredients. Fold in half the dry ingredients, then the remaining bananas, then the rest of the flour and spices. Stir in a big handful of chopped nuts and a handful of raisins or cranberries. Bake mini loaves for 24 - 28 minutes, rotating halfway through. Muffins take a few minutes less. A large loaf will take 50 - 60 minutes; I would reduce to heat to 350 after 10 minutes or so.

Hope this helps. ;o)

P.S. The same ratios can be used with unsweetened applesauce (to which I always add a chopped, unpeeled tart apple), or pumpkin puree. I typically add a bit more buttermilk with the pumpkin, and sprinkle with cinnamon sugar before baking, as the pumpkin is much less sweet.