Bottom line for me is my Netflix HD is, subjectively, usually pretty awful with blatantly obvious compression artefacts. If I were to rank it out of 10 I’d give it about a 3, whereas BBC1 HD on Freeview I’d give a 6. On my HD Apple TV, I’d give regular iTunes movies an 8, with Apple TV+ getting a 9+.
Then again, I only pay for Netflix HD at the moment as, last time I looked, their 4K seemed to be largely restricted to their own home made series, that I rarely watch. As suggested earlier about a different service, I wonder if it might be a policy decision to allow HD to look bad, in order to make 4K look better?
I only noticed yesterday, if I touch the ‘info’ button in my TV remote it shows me the bitrate and resolution. The bit rate is static, does not fluctuate, so I guess it is an average. Watching a 2017 movie in HD, it showed 4.6 Mbps all the way through.