I have been using unity for somewhat around 6 years now. I’ve been using it for games only as a hobby, yet had an idea for an app and started wondering. Is unity useful for developing mobile apps (not games)? What could be the main downsides of doing so? Thanks in advance for all the anwsers 
Unless your app needs complex 2D/3D graphics, I’d quite strongly recommend against it. Yes it’s possible to do, but the biggest downsides will be:
The performance and battery consumption of the app will be much worse. Native apps are much more efficient to for various reasons, and they typically only re-render when something changes on screen (which saves battery).
The unity UI system is designed around the types of UIs that are typically needed in games, meaning it doesn’t scale very well for “application” style UIs. Once you get familiar with pretty much any modern UI framework (e.g. React native, xamarin, etc), you’ll never want to go back to Unity.
Other things like build time, download size, and compatibility with older devices will likely also be worse.
The only scenario where I think unity would be worth considering is if (a) you’re not too worried about the quality of the app, and (b) you don’t have time to learn a new UI framework or deal with the (relatively manageable these days) complexity of handling multiple platforms (iOS/Android).