Think about Background Location Tracking when the app is closed. I saw some video on YouTube in which the speaker has one such use case like he has a Car Service like Uber. But the Driver needs location tracking & the app shows the path to reach from point A to point B. So if the app is running all the time even when the car is stopped at some place. Lets suppose the Driver wants to take a piss or have food. Then he will stop so the app won't be running now. This specific use case with React Native will run a lot of the battery & CPU cycles as the speaker told in the YouTube video. The original video I could not find. But I think I explained it perfectly now.
Summary of the usecase if I have not explained well - Uber like Car for picking packets from one place to another. Must be keeping track of Driver Location. But should not run all the time, must only run while Car is moving. This won't work with React Native, I mean it would but it takes a lot of performance, CPU, battery, etc...
Another 2 videos also show where React Native cannot be used for particular usecases like https://www.youtube.com/watch?v=HXKFQu2cP4c & https://www.youtube.com/watch?v=psZLAHQXRsI
Let me know if its clear now ?