Doesn't Unreal require you to code in C++? That's a big reason why I'd avoid it when I don't require maximum performance.
Blueprints are actually really cool for what they are and have a bunch of interesting debugging considerations, such as being able to see the flow of "code" visually.
Either way it's not something I'd trust enough to use for a commercial project, so I definitely wouldn't say UE supports C#.
This is mentioned on the linked site:
"You will need source access to Unreal Engine on GitHub to get access to the MonoUE fork".
I wouldn't go around telling people UE4 supports C# based on this.