The primary reason PKG files are "better" is that they are the native format used by the PlayStation 4 itself. Unlike an ISO, which is a raw disk image that requires a "wrapper" or an emulator to translate data, a PKG is essentially a container that the PS4 operating system (Orbis OS) already knows how to read.
While "ROM" is the catch-all term for game backups, the is a far more sophisticated beast. Between its native stability, its ability to bundle DLC and updates, and its superior compression, it is objectively the better way to manage a PlayStation 4 library. ps4 pkg roms better
For those using homebrew environments like GoldHEN, the installation process for PKGs is the peak of user-friendliness. Once your console is exploited, you simply place your .pkg files on the root of an ExFAT-formatted drive, plug it into the PS4, and use the in the settings menu. The primary reason PKG files are "better" is