How is a PhD by publication valued in the US?
I think that you will find that such degrees are accepted at face value just as you would expect. They are different systems and different ways to guarantee quality, but don't represent any difference in quality as compared to the US model. In fact, having those publications reviewed by independent (journal) reviewers is an added guarantee of quality. 

But if the faculty of a respected university has granted you a doctorate in Europe, you can expect it to be valued in US.