Quick to get some sort of app up and running
Lots of vendor lock in (even after detaching/ejecting), doesn't teach you how to truly deploy a mobile app
There's lots of cool things that Expo is doing, but I'm not sure this should be presented as a full-scale mobile development platform. Even with the eject scripts, there's still a fair amount of vendor tie-in that you get from leaning heavily on a kitchen sink of an native app that bundles every framework and has it's own set of abstractions. If Apple/Google releases a new iOS/Android API that you want to use, you have to wait for Expo to implement it first.
There's also something to be said for the large amount of abstraction of platform-specific standards. I don't think Expo does a great job of building components that feel at home on each platform, starting with their recommendation of using a non-native replication of UINavigationController in `react-navigation`, to loading bundled fonts asynchronously, to using FontAwesome icons over the iOS-provided UIBarButtonSystemItem icons.
If you're building a one-off mobile app that has no plan on being maintained, Expo might be the right fit for you. Looking to build a mobile app that will be maintained for some time to come? Stick to a vanilla React Native or native Swift/Kotlin app and learn the platform first.