[dawidpotocki]

GPU video encoding is pretty much always optimised for real-time encoding, meaning that it can't run certain optimisations as it would increase the time to encode.

Compare x264 veryfast and veryslow presets. There is a quality difference at the same bitrate.

Additionally, GPU encoders don't have as many psychovisual options as CPU encoders as they would need to be included in the hardware and adding extra options to CPU encoders is much faster, easier and cheaper.

You could build a non-realtime GPU encoder, but there is not much point.