Yeah, when you're compiling from source code, it's much easier to add in ARM support. Kernels do need to handle more hardware-specific stuff, like interrupt handling, context switching, feature enablement and the like will likely need custom ASM code and might have different parameters/events to handle. But at the user level, you can often compile for ARM by just changing a few command line arguments and it'll be fine as long as you don't rely on inline ASM and have ARM versions of any libraries you need.

You still might need to do some adjustments for specific behavior differences when it comes to concurrency and atomicity of operations. It didn't surprise me to see that the previous attempt to make an ARM ReactOS didn't support multi-threading, because trying to enable it was probably an unreliably buggy mess with race conditions all over the place.