I've brought various apps, bundles, and frameworks from PowerPC to Intel to 64-bit to ARM ever since macOS 10.0 first launched. Usually the most difficult parts were:

1. During the PPC to Intel transition, converting code that expected all data to be big-endian over to handling little-endian data, and catching integer division by zero before sending such operations to the CPU
2. During the 64-bit transition, switching from all the APIs Apple removed over to newer APIs, if not already done, and converting all code that expected integers and pointers to be 32-bit over to 64-bit
3. During the ARM transition, converting code that abused variadic functions to code that used them properly, and converting all code that expected long doubles to be 128-bit over to 64-bit (I know some developers were burned by the VM page size change, but that didn't affect anything I did)

But yeah, usually the most difficult part of the transition is managing the dependencies. Whenever Apple transitions CPU architectures, if your app depends on a closed-source third-party library or kernel extension made by developers that went out of business years ago, you're more or less screwed unless you can find or build a replacement.