The previous Assassin's Creed game, Odyssey, ran much better on Nvidia hardware than it did AMD hardware. An RX 580 was massively behind a GTX 1060, for example, despite being generally the faster card, and a liquid-cooled Vega 64 couldn't even keep up with a GTX 1070 (with a standard one being barely faster than a GTX 1060). I assume you were similarly-outraged about that unacceptable level of performance on AMD cards?
There are always going to be games that prefer one architecture or another to varying degrees. That's just how it's always been, throughout the entirety of PC gaming history.