I worked on at both companies on drivers. The programming models are quite different. Both make GPUs but they were designed by different groups of people who made different decisions. For example:

Nvidia cards are much easier to program in the user mode driver. You cannot hang a Nvidia GPU with a bad memory access. You can hang the display engine with one though. At least when I was there.

You can hang an AMD GPU with a bad memory access. At least up to the Navi 3x.