Unfortunately, flat design has a major flaw: it often leads to click uncertainty and decreased user efficiency. When designers flatten the UI, they tend to remove many signifiers that normally tell users where to click.
Flat design, especially when combined with strategic minimalism, can be a powerful aesthetic tool. It can convey a sense of luxury or trendiness, and in some cases can be used to appeal to young adult users. It’s a lightweight UI, and can be easier to support across a range of device sizes.
Users often leave Web pages in 10–20 seconds, but pages with a clear value proposition can hold people's attention for much longer. To gain several minutes of user attention, you must clearly communicate your value proposition within 10 seconds.
A bad website is like a grumpy salesperson.
Unfortunately, flat design has a major flaw: it often leads to click uncertainty and decreased user efficiency. When designers flatten the UI, they tend to remove many signifiers that normally tell users where to click.
Missing Reference