It’s mostly the immediates, I mean, how could I not mention the immediates. The other parts are indeed arranged fairly straightforwardly, but trying to read off a 5-bit register field crossing a byte boundary when the instruction is written in little endian—or even just a three-bit field somewhere in the middle to figure out what the instruction even is—is less than pleasant.

Again, I recognize this makes things simpler, not more difficult, for the hardware, and even a compiler’s emitter will spend very little code dealing with it. But it’s not easy on the eyes.