> Again the fashion for extreme flatness, with square icons and buttons that disappear into the background are shown to be counter productive and user hostile.

Any sources for this? I tend to share this sentiment, but have never really read too deeply into it.

> They might "look nice" to some lead designer, but they don't work as well.

To further this, I think it is important to acknowledge too that these design decisions are not made exclusively because they "look nice." There is value in "sameness" when you are competing with other products/brands/sites that have already established a certain style, convention or pattern. After all, we learn and orient ourselves by subconsciously comparing new experiences with ones we've already had. So when you experience similar patterns in a new interface, it drastically decreases the cognitive load in learning how to use it. To your point though, there is a fine line in how effectively you can push simplicity, and there are many subtle visual variables beyond the reductionist square button that could help to improve legibility of elements as distinct components that are largely ignored in current trends.