> The target processor is not that important from the users point of view
It's important for the operating system and the processor was used for very different machine hardware: standalone workstations, headless systems, embedded in a Mac, embedded in a SUN, embedded in a network switch, ...
> Lambda and CADR shared the exact same source code, and microcode.
But not the 3600, which was a 36 bit architecture and not the Ivory, which was a 40 bit microprocessor. A lot of the core routines were different on a 3600 and an Ivory machine. The VLM was even different, running on a 64bit microprocessor, implementing the emulator in C and assembler. There is low-level functionality which has two or three different implementations.
The Ivory machines could be very different from a LAMBDA and a CADR, for example as an embedded CPU in a very different host machine (Mac & SUN). That had a lot of consequences for the operating system.