I converted a photo to HEIF and JPEG with various qualities, and it is quite clear that the compression from HEVC is superior to JPEG for high resolution photographs. At least subjectively. I have not done a detailed objective analysis.
Here are the photo and its variants (©2009 Keith Cooper northlight-images.co.uk)
The JPEG files have file-Jxxx-y.jpg where xxx is the "percent" quality in the save setting (100 is best), and y is the bit.
For the HEIF files it is file-HCRFxx-y.heic and xx is the CRF value (0 is highest quality) from the X265 codec and y is the bit.
The original file is the 16 bit TIFF.
Maybe another photos would create different outcomes, but it was the first usable I found when searching. About the bit 8 = yuv420p and 10 = yuv420p10.
I used Apple Preview to save the JPEG so I there maybe better software which compresses better.