If you have a good convection oven, you don't need an air fryer. Just use a perforated cooking sheet so the air can fully circulate and make sure you line the area underneath with a large tray or foil to catch any drippings. The plus is that you can put a ton of stuff on without stacking (which kills the air fryer effect) and it's WAY easier on cleanup.
I've got a Bosch convection oven and the results are exactly the same as the Ninja. Cooking times aren't that much longer, in my experience, but that likely is a factor of how good the convection feature of your oven actually is and, most importantly, put the foods cooked in the exact center of the oven.
I do find the air fryer is better/faster if I just want a quick snack. But for meals for more than 2 people, the oven works better as I don't have to work in batches.