There are two schools of thought on app backends: Backend-as-a-Service (Baas) and do it yourself.
BaaS is great for people who just want to focus on app development, or get something out quickly - you're paying for the convenience of not having to do it on your own, or learn a whole new part of development.
Doing it yourself using Python, Rails, PHP or something else is great for people who want to learn more about this part of development, have a backend that suits their purposes exactly, or are hoping to build a much more substantial application on top of what they have, and need a strong foundation that will last through multiple iterations.
We are not a judgey company, and there are benefits to each option, and drawbacks, and you should definitely choose whatever fits the needs of your company/app/amount of free time :)
That being said, with the recent release of Rails 5, we wholeheartedly recommend Rails for DIY backend. There's a really great part of this updated framework called API-Only Mode, and it looks like it was designed specifically for mobile developers. This is the official documentation on how to make an API-Only Rails 5 app that you can attach to your iOS app, and this is a blog post that's a lot of the same information, but slightly more newbie-friendly.
If you'd rather use a service than spend time to sort it all out on your own, iOS Dev Tools is a great resource for comparing providers, and finding the best one for your app's needs.