Video encoders can vary widely in quality. There are lots of parameters that allow for a wide range of "correct" encodings of the same source video file. Realtime hardware encoders in general have lower visual quality for the same bitrate than software encoders that may be slower but more thoroughly search through different options for encoding each group of frames.

Decoding is much more deterministic, so speed and power efficiency are the main ways hardware decoders can differ.

Encoded media either comes out blocky, with artifacts, or plain old slow. Some also have bugs related to the encoder that app developers have to contend with.

The h265 quality is just not up to par. The av1 encoder does a nice enough job, so does the h264 one.