These issues include crashing and poor performance on machines that ought to be able to run the game absolutely fine. The vast majority of the community has put the blame on Ubisoft's choice of DRM, but according to one fairly experienced modder, however, this is very much not the case.
Kaldaien's GitHub post is pretty self-explanatory: "It has a much too conservative resource ceiling for textures. The engine loads, then unloads and later reloads the same assets hundreds of thousands of times over the course of an hour. All the while, VRAM is only ever 25% filled on my GTX 1080 Ti and none of that nonsense is necessary."
He compares Odyssey with Arkham Knight, which had lots of optimisation options due to it being built on Unreal Engine. Sadly, there's no easy fix for the problem in Assassin's Creed Odyssey, and the modder had to resort to a little-used Windows feature (Multimedia Class Scheduling) to raise and lower thread priority according to system responsiveness.
Said modder's main takeaway here is that Ubisoft needs to work on the massive driver overhead that their games seem to have, and which is caused by "naive resource management for such a vast open-world game. Let's wait and see if Ubisoft do anything about this issue in future patches.
Have you checked out these Assassin's Creed Odyssey Mods yet?