Hi.
This is like a question that most of the people doesn’t have an objective answer, most of the people that worked with code since a long time ago haves a hard time to admit that BluePrints works fine, I worked in c# since forever and for me also took a loot to admit that blueprints are good.
But let me hear you guys, please can you tell me a CONCRETE example of something that you CAN’T do with blueprints, and the only way to do it is with c++ code?
I’m not gaming developer, just a very senior software developer, i’m fooling arround with unreal doing some stuff, so for me this is all new, but I have a strong knowledge about coding. So hit me with your best example 
One example is reading pixel data from a texture on the CPU. You can always just MAKE functions that exposes the functions you might want in blueprints and voila, you have them.
I have yet to find a feature that either did not already exist in blueprints or that I couldnt expose by writing a C++ wrapper.
HTH
This is great question to talk about on forums, but not here.
And I think there are 100500 similar threads already.
There is no need for a Concrete Example. Blueprints can do what is exposed from C++ and any Combination of the exposed stuff and not a single thing more than that. And there are a lot of things not exposed that you get for free in C++.There are other Limitations but they will be obvious to you once you need them =)
But if you need a Concrete example that was not possible to use until very recently are TSet and TMap inside BPs and they still have a couple unresolved Bugs.
Extending the editor itself is a pretty huge example of C++ superiority.
TArray has a Build in Binary Heap but not exposed. (A Example that you get for free inside C++ lots more I dont want to start a list)
Extending the Editor UI with Tools not possible.(Asside from Blutility and Construction scripts)
Templating, talking to the Hardware, talking to the Render Thread, Multithreading, Custom Asset Pipelines, Third Party Librarys communication that are not provided by the Engine, a lot more things related how you Handle meory and Algorithms efficently. etc. etc.
Issuing direct commands to the hardware through OS level function calls etc. Blueprints is just interpreted and executed as it is parsed.
What do you mean with extend editor? it´s like using resharper in c#? third party tools and so?
You seem to have experience on this, what do you mean with Talking to the hardware?