Using Xcode Targets
Source files must be added to all the framework targets by hand, same goes for any new files you add. Since this is very easy to miss doing from the new file dialog, it is generally discouraged to add the same file to multiple targets (if what the targets are building are not significantly different). You not only have to maintain the code for the frameworks individually, but also all the build settings for them. This plus adding tests for all the targets quickly becomes an unreasonable goal to set for any development team without breaking all the frameworks into separate entities -- and away from a singular code-base.
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While changing anything in a working build system is a risk, there are some big benefits you get for using xcconfig files instead of build settings stored on the project file.
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A scheme can specify a single build configuration to be used for multiple targets. This means that instead of having schemes for each framework and then each application target, you can have a single scheme that tells a single framework target to be built using a specific configuration and associated xcconfig file (you can set this up to be able to change the requirements of building based on target OS, so a single target could build for iOS, Mac, watchOS, and tvOS) before building the application target. Doing this means cutting the footprint of your code down significantly as well as removing the complexity of managing multiple targets.
