On Apple Silicon, writing to memory on a PCIe / Thunderbolt device will generate an exception. ARM spec says you're allowed to write to devices as if they were memory but Apple enforces that all writes to external devices go through a device memory mapping[0]. This makes using an external GPU on Apple Silicon[1] way more of a pain in the ass, if not impossible. AFAIK nobody's managed to write an eGPU driver for Apple Silicon, even with Asahi.

[0] https://developer.arm.com/documentation/102376/0200/Device-m...

[1] Raspberry Pi 4's PCIe has the same problem AFAIK