The editor GUI is very powerful and intuitive. It allows pausing gameplay and manipulating the scene at any time as well as progress gameplay frame by frame. It also has powerful asset management and attribute inspection. This allows it to be more powerful than other, simpler drag-and-drop engines...
Performance can have real issues with 2D games. Since Unity is actually a 3D game engine, it drags a lot of overhead with because of this. For example, for mobile , one could never achieve the performance of more specialized 2D engines with Unity.
The 3.0 alpha version is basicly impossible (for now ) to get in to without a proper tutorial (be it video, article, etc.) and because the 3.0 v is very new and not even stable - it doesn't have too much video tutorials
Because both Core and Editor are completely plugin-based, Duality not only incorporates a clean and modular design, but is also very extensible - even if it isn't Open Source. In fact, most of the standard editing capabilities come in form of Editor plugins.
As of writing this Godot 3.0 (The first version supporting C# , VisualScript, etc,) is still in alpha 2. So that means that the documentation is not ideal and not as simplified as some people would like it (Let alone the massive lack of video-tutorials).