Just to add another perspective, as an American living in Germany, I often find that the German criticism of American optimism/interpersonal warmth is extremely paranoid. I have heard so many Germans describe American "niceness" as "fake", but I don't think the American approach is rooted in dishonesty as it's sometimes assumed by outsiders. For instance, Germans will be shocked if they go to the US, and a stranger starts a conversation with them waiting in line at the convenience store with a lot of warmth and curiosity. As I understand, to them this reads as the approach of someone who wants to con them or trick them, putting on the guise of un-earned closeness. But Americans in my experience just give interpersonal warmth a bit more freely, and are more willing to have friendlier interactions with someone they don't know, with no expectation that the relationship will last longer than the time both of you are standing in line, while that type of warmth and friendliness will be reserved for close friends and family in other cultures. As someone who grew up in American culture, it's not "fake" or forced when I smile to a stranger, or congratulate them on the new grandchild they just told me about in our first meeting. It's just part of the culture, and it's something which I enjoy to give and receive in these random, short interactions throughout the week.
And when it comes to work criticism, I agree that there is some value in what would be considered "blunt feedback" by American standards, and that Americans are sometimes too hesitant to give it. At the same time, I think this also comes from a different cultural approach which is also valid. Americans have deeply rooted ideals for independence and self determination, and a general sense of optimism. I think the default position when someone is showing you a piece of work is often to assume that they have it under control, and that it would be presumptuous to tear down a piece of work someone else owns and that you are seeing for the first time. By focusing on praising the best elements of the work, you are giving your colleague feedback on what they should focus and expand on, and you are leaving it up to them to discover the flaws in their work and resolve them in their own way. So you would reserve direct criticism for times when you think there is a critical misunderstanding in the basic direction of the work which will prevent the correct result from being reached.
And you can cliticise the American approach all you want, and I will be the first to admit that it does lead to a lot of problems and blind-spots. But as someone who has worked in the US and in Germany, my experience is that American companies move and innovate a lot quicker than German ones by focusing on potential rather than flaws, and that trend seems to have been borne out if you look at the major innovations which have come out of each country in the last 30 years.