John here, one of the app engineers at Substack. Our team has lots of experience with Swift and values the level of polish you can achieve natively, so went with that. We try to avoid third-party dependencies so native implementation and customization of UIKit, URLSession, and CoreData are important to us.

Cross platform frameworks like React Native, Flutter, Xamarin, etc are super interesting but in our experience can hold an app back long term. As you're able to dedicate more resources to the product, it's more likely that you'll want custom UI (animations, transitions, etc) and functionality that require full access to native APIs. It's a tough decision to make for early stage start-ups but we also went with native at Cocoon (our last startup which was acquired by Substack). I think some important factors that should go into the decision are 1) how important you think both Android and iOS support are out of the gate 2) how soon you'd expect to be able to hire native engineers for both platforms 3) your long term vision for the product and its desired complexity. We love Android (I've got some Kotlin experience myself) and will have it out ASAP, but with the existing email/web product there is less pressure for cross platform in our case.