Some folks tell us that curves are perceived as something pleasant and harmonious, while straight angles are seen as sharp and dangerous. But since we're not interested in Satanism and horoscopes, let's consider the practical aspects of both approaches.
I purposefully don't touch upon industrial design, because there, curves are a separate form of art, linked not so much to harmony as to the figures in the manufacturer's invoice.
Shapes with straight angles tend to group together more tightly into a cohesive structure, as they better maintain connections between each other. Rounded rectangles will always remain autonomous.
For verification, use modules as a mask for the photo. Even with strong gaps, it's easier for the eye to see the whole picture in the left example. In the second example, there are more questions than harmony.
This is also clearly visible in the navigation module. With a small gap in the first case, the arrow still belongs more strongly to the inscription, while in the second case, it becomes almost an independent element. Even if you remove the gap and color difference in the second option, there is still a sense of two entities.
At the same time, there's an interesting technique where external roundings unite differently colored rectangles with sharp angles into one object. And this union works even stronger if you just leave two sharp-angled rectangles side by side.
The reverse technique doesn't work as well because it requires spacing.
