I have a Ninja Foodi 9-in-1 and yes, you can definitely bake a shop-bought pie in them. I love mine as it does so many different things, but you need to consider what you want to use it for, as there are so many different models.
I have the 9 in 1 model too (I think!) & bought it off QVC some time ago. Love it & use it all the time for everything BUT it’s impossible to keep the inside walls clean 😱. Why they didn’t use a non-stick coating is beyond me! I’ll carry on regardless as you can keep other parts near the food clean, but when I can’t bear the look of it anymore will dispose of purely because of the cleaning issue.
I have a Ninja 14 in one, I absolutely love it, you can cook a whole chicken in it, even bake bread. The only times I have used my main oven, were for roasting a turkey and batch baking. There are already a few threads about this on GN.
The main thing is how many you are cooking for and what do you cook - I only use my oven when family come or if I'm making a full Sunday roast, and occasionally if I'm baking. I've had to buy some smaller dishes to use in my Ninja as I never used to cook for two. I do agree with another poster about it getting dirty though - the basket and pot can be cleaned but the lid is a greasy mess behind the element.