One of the key determining factors of the quality of a GPU renderer is whether the resulting image is the same as it would be when rendering on the CPU. For almost every case, Fractorium will produce identical results on both, however there are a few corner cases where there are differences.
This happened to be the only variation where I was unable to get the outputs to be the same. In practice, the GPU results look better for hexaplay3D.
If you find any other cases where the outputs are different, then this is a severe bug and you should report it to me immediately.