Unity isn't just closed source, but your main access to the engine is through an ancient Mono .NET environment that hasn't been updated in years due to licensing issues. The latest re-licensing of Xamarin should help here, but as with many other features Unity will be afraid to make upgrades for fear of braking compatibility.
This means that most of the Unity game code that will set your game apart has to be written in C# 3.0, and your memory use and performance is heavily taxed, unless you resort to platform specific native plugins. As a game developer this scales down your ambition down to mostly casual games. The type of AAA games that Unity advertises are mostly distinguished by large amount of content, but nothing innovative in gameplay or technology besides what the engine provides. I can't imagine anyone starting a No Man's Sky or even a next Minecraft in Unity.
This is sad, because about 5 years ago I was cheering for Unity and having a lot of fun with some prototypes. They know they are a mill for casual clone games too, since they are shifting their business focus to online developer services like advertising and multiplayer hosting with per-player fees.