Downscaling an image using Export As produces blurry and stepped images. Save for Web produces clean and smooth images. This is using the latest version 21.1.1 on Windows. This image was 4500x3000, I've downscaled it to 1500x1000 in both Export As and Save for Web.
Just look at the attached images and see how the word "Cover" looks so much worse in Export As when compared to Save for Web.