I don’t know C# so I can’t say how fast development is with it. Going from what I have learned from UE3 to UE4, I am happy that there is both blueprint and c++. I would never use blueprint only to make a game unless that was a very very very simple game. Even then I would be tempted to make my pawn, controller and game mode use a c++ base class in case I needed something that is not exposed in blueprints. However with a BlueprintFunctionLibrary you can workaround those issues most of the time.
To say that blueprints are a toy seems intentionally antagonistic to me. It undermines the work of developers who have used blueprints and basically calls them stupid for using them over other options. All I can say about that is that you are wrong to demean blueprints in such a way, both in a professional and logical manner. It reminds of the old argument between programmers in chatrooms that went something like …
I use Javascript because it’s fast.
Javascript is rubbish. I use Java.
Java is rubbish. I use visual basic.
VB is rubbish. I use C++.
C++ is rubbish. I use C.
C is rubbish. I use Assembler.
… and it goes on until one elitist with self-absorbed opinions ragequits.