Is that really true? I would expect that going from IR to native code would be the hardest part because you have to worry about architecture and supporting different ISA’s. IR can be simple because it is abstract and hides lots of the dirty details of the hardware.

Generating code is not all that hard. Generating fast code is a challenge. Decoding instructions on some architectures is hard. If you have a nice general IR naively generating native code can be just a table lookup with a register tracking/allocation routine.