I'm react-native plugin developer, but still struggle to understand the value proposition of Expo. Or maybe proposition is OK, but real life experience is miles different. I think most projects very soon get to the point, where they need to be ejected from Expo, because of need of external react-native plugin.
I see it more of a fast start to generate an MVP type of product because their are certainly still flaws in expo but the whole idea of not worrying about module updates breaking your app are huge. I personally use and like both but for different situations