Child's play compared to what you'd need to do on a modern chip.

I don't think it's the chips, but the operating environments. Modern CPUs offer dozens of multipurpose registers and many more instructions and addressing modes compared to those old, low-cost CPUs, which should make things easier, not harder. But no-one's building old-style dedicated systems around modern CPUs; our code now has to play nice with firmware, OS, libraries, and other processes, including resource management and preempting.

Compare a single-gear go-kart to an automatic sedan. Getting top performance out of the go-kart on a closed track is difficult and requires nuance. If we could drive the automatic sedan around the same closed track, we could easily demolish the go-kart, and not just with raw engine power. The improved acceleration, braking assist, and power steering are enough. But when we drive the sedan we're usually doing it on public roads with traffic signals, intersections, speed limits, and other road users. That's what's more difficult.

It's the machine language monitor on the 40-column screen of the Commodore 128 (or, more likely, an emulator of the same). I had a whole part about that, BASIC DATA statements full of numbers, and about how anyone with any sense actually used an assembler even back then in an original draft of my comment, but decided to keep it brief.