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Why does GPU need to trade off quality for speed? GPU is faster than CPU, that is the whole point of GPU encoding, isnt it?

Does it mean just that nVidia and AMD engineers done a poor job when implementing those encoders?

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No. Decoding is a job mostly done by specialized hardware - the shader units are used sometimes, before a fully fixed function implementation is ready. Encoding in particular doesn’t map well to GPUs. They can do it, using varying degrees of fixed function and shader cores, and it’s nice to isolate that load from the CPU, but they implement fewer of the analysis, prediction, and psychovisual optimization tricks that x264 and x265 use, and less of the optional features of the format. They often can beat software at specific, fast speeds with lower power consumption, but the trade-off is being inflexible and unuseful for making encodes that are transparent to the source.

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