I just finished testing the new h.264 hardware acceleration also. My source is Sony a7s II 1080p footage. I output high quality 30 mbits per second, both hardware accelerated and non-hardware accelerated.
I had the same results as you. The hardware accelerated h.264 output is MUCH lower quality, fuzzy, lack of detail and easily noticed even at high bit rates.
I tried a 2nd test. I added an adjustment layer to do some minor color correcting, This was much better, but still not quite as good as the software only.
I tried a 3rd test. During export I clicked the "Maximum Render Quality" check box. Again, much better, but still not quite as good as the software only.
These results actually do make sense. Hardware encoding has long been inferior to software encoding.
This feature is still under development and there are some limitations to i as of now.
One limitation I can mention looking at Matthew Tester's screenshot of Export settings that it is supported only for Level 5.1 and lower.
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I understand it's still in development, I'm struggling to find any detailed feature information though to give me an idea of what I should actually expect or how I should be using it. For example I did not know about the level 5.1 limitation. My original exports were using this but I raised it to ensure I hadn't missed anything.
I do not believe this is GPU related at all as I have tested it with the GPU acceleration turned off with no difference.
I have tried disabling key frames and had the same or similar quality improvement, how bizarre that it should make such a difference. Thank you Andy for testing this and posting your results.
Edit: Further testing shows that the quality is improved further by using level 4.0 with key frames turned off, however this limits the framerate to 30fps. Level 5.1 at 30fps is still awful.