What should I look for in a Shopify mobile app builder?

Three things: whether checkout stays Shopify's, whether the app is genuinely native rather than a wrapped website, and whether the store accounts are in your name. Everything else is preference.

Those three decide whether you own what you paid for. The design, the features and the price are all negotiable; those are not.

A fourth, quieter one: ask to install an app they have built. A studio that cannot show you a live app is asking you to be the first.