You should not have any problem running that game unless it is a game engine issue. In fact it probably would run better on that card at high settings than low. I don't know what resolution you are playing at? As long as it is 1440p or lower and not 4K you should be able to get acceptable frame rates. I don't play that game but I do play many other Ubisoft titles such as the Far Cry series, on my RX 580 with zero issues. It may also be that this game is just a buggy game with engine issue.
I did just searched the Steam forum for that game and tons of people complaining of the same issues you are complaining about. They were mostly on Nvidia cards so this is really looking like it is a game engine issue.
You might message Ubisoft and ask them if they can suggest in game settings that would best fit your card. Other than that, you can do trial and error. Disable one at a time things like anti-aliasing, post processing, ambient occlusion, etc until you find the setting that makes a real change.
If none of that helps,
Maybe you could reply back with complete system specs:
When posting a new question, please provide as much detail as possible describing your issue making sure to include the relevant hardware and software configuration.
For example:
- Issue Description: [Describe your issue in detail here]
- Hardware: [Describe the make and model of your: Graphics Card, CPU, Motherboard, RAM, PSU, Display(s), etc.]
- Software: [Describe version or release date of your: Operating System, Game/Application, Drivers, etc.]
