Stitching artifacts while rendering large volumes AMD GPUs can be solved by adjusting the number of partitions by copy-pasting a few lines of Python code as shown here:
I would recommend to go with NVidia if you need to render large volumes. Either AMD drivers are lower quality, or maybe NVidia drivers are just as bad but VTK is tested more with NVidia and so VTK contains more fixes or workarounds for NVidia bugs.