The question is:
LuckGrib is available on both iOS and macOS, and the application has the same name on each platform. If I was to buy the app on iOS, would the purchase transfer to macOS? (Or the other way around?)
The short answer is: no. If you want to run LuckGrib on macOS and iOS, you need to purchase the application on each platform.
Although the app has the same name on iOS and macOS, they are completely separate applications.
The two apps are similar in many ways, but they also have many differences. Everybody seems to agree that the iOS app is easier to use, and it also comes with some built in tutorials, which can be useful.
The macOS app has more features, which may or may not be useful for you, depending on how you use the application. For example, meteorologists using the macOS app have the ability to extensively customize the draw styles. If you are happy with the parameters currently supported in the application, you may not need this ability. Sailors may also prefer the macOS app as it provides slightly more text information in the weather routing system (although the reports that are generated, the actual routes generated and the visuals are the very same between each platform.) The Mac version of the app also supports a manual routing option, which while complicated to use, can be useful when working with very long passages. Weather researchers may enjoy the ability to import huge files on macOS, as Mac’s can be loaded with lots and lots of memory.
There is a considerable effort needed to implement and maintain each application. The author does not cut corners in the app development - a great deal of care and attention to detail is exercised in the development of both of these applications.
If you are enjoying the app on one of the platforms, consider also purchasing on the other. You can think of this as a way of showing gratitude to the author for all of the work that has gone into their creation.