On Mac and Windows, you are relying on Drivers to do the work where you dont have the source code. While the piece of code is running on OpenGL or Direct X or Metal. And on top of all that there is a composition layer.

Modern GPU drivers is an insane piece of optimisation / engineering. But the complexity means you will never quite grasp the problem when things go wrong. In the early days of Firefox or Thunderbird it is recommended you turn off Hardware Acceleration whenever you see Font or Graphics rendering problem. While a lot of these have been fix ( or migrated ), it is still popping up from time to time.

There was a recent thread on HN ( I cant find it sorry ) Firefox asked for help on a GPU rendering bug that they have a hard time figuring out what's wrong.

On iOS, Apple owns the whole thing they can figure out what's wrong and fix it. As Apple write their own GPU drivers.

The GPU is a whole separate computer, running code independently, with its own clock scaling and power management. Having two separate things that can break is more complexity than one thing that can break. And having to have the two communicate in order to do delicate procedures like waking from sleep increases complexity dramatically.

Also, on many Macs, it's actually three separate things. There is the CPU, the integrated GPU, and the discrete GPU. The system switches between the two GPUs dynamically based on load and whether the system is plugged in.

How's that different from each CPU core?

The CPU cores are all (more or less) identical. The GPU has a completely different architecture.