The photos are neither low-quality nor highly-compressed. I personally can't tell much of a difference 95% of the time and I'm a technical person. Here´s a comparison and a closeup, left image is the original, right one compressed with the Unlimited Storage setting:

full picture https://images.huffingtonpost.com/2016-01-01-1451631423-7665...

cropped detail https://images.huffingtonpost.com/2016-01-01-1451631515-2060...

Also, I believe Photos storage is unlimited even in Original quality if you own a Pixel phone, which may or not may be the case, but still.