I don't know about all of you, I don't like these new flat icons that everyone is using. What ever happened to the old icons, like on iPhone and Samsung they used to have them years ago. Those were good times. Now it is always these stupid boring cartoonish designed icons. Side note: Somebody please update this icon pack. I am trying to use it on xfce on arch but some of the icons aren't working properly because it hasn't been updated in a while. I'll donate to you right away if you do it. Link to the repo: https://github.com/madmaxms/iconpack-obsidian
Interfaces can be needlessly complex regardless of being flat or skeuomorphic.
But flat interfaces still require mental effort to parse. Especially when the interface is complex and/or crowded and you're trying to pick out active UI elements amongst decorations like group boxes/panels.
Essentially, flat interfaces are currently popular because of touchscreen devices. Touchscreen devices have limited space and thus need simplistic UI elements that can be prodded by a fat finger on a small screen.
But I don't need a flat touchscreen-friendly interface on my non-touch dual 24" monitors with acres of screen real estate. I need an interface that nicely separates usable UI elements from the rest of the application window. That means 3D hints on a 2D screen, which allows my monkey-brain with five million years of evolved 3D vision the opportunity to run my "click the button" mental command as a background process.