I think maybe you're not looking outside the box. For example. Yesterday I downloaded a piece of software that was supposed to help me in crafting for another game. It was a Windows application. Now what I expected was something written in a 'normal' language with a 'normal' interface. To my surprise, this person created a desktop app designed for calculation in Unity? To take this in another direction, the reason I purchased C2 years ago was to augment another piece of software I had written in Delphi. I was able to take C2 and make an Android based teleprompter. So I'm not sure what your definition of 'serious' is but both engines have the ability to get just as serious as you wish. You can delve into C# or JS in Unity to make things work exactly as you wish and you can use JS in C3. If I can get the server tech working right, I'm in the process of building a game that, in theory, could host thousands of players... in C3. To me, that's pretty serious.
When I look at Unity I see an utter mess. For example, I read a tutorial on how to do something specific with Unity. What this skilled programmer did was attach 15 odd scripts to a whole host of objects creating 15 different places where something could go wrong. Debugging that was a nightmare. Was it structured? Oh yes. Was it a nightmare to follow... Oh yes. I rewrote the entire thing into one C# script so that I didn't have to play hop-scotch trying to get it to work. Now if this is how the magic of grouping assets works then I'm not impressed.
Furthermore, every single time Unity comes out with a new version, half the games that use it break. Read some of the dev blogs for games that use Unity. They consider it a major accomplishment to upgrade their code to the latest Unity version and those that don't succeed end up reverting back to a previous version. And that's damn near every version that Unity releases.
No one game engine is going to create every conceivable game. And even when you do find an engine you like, it's going to take some creativity in order bend and shape it into what you need. Many times, you'll be working with the limitations of the engine itself. Finding ways around those limitations and still accomplish your goals is what creating any software is all about.