Gumax, That’s a good question, and one that’s a bit complicated to answer with a hard percentage. It’s maybe better to describe why some things are exposed and others aren’t. BPs are primarily meant for artists and designers to quickly implement functions and classes that are developed using C++ for performance and scalability. So, lots of properties and functions are exposed to ensure that designers can make network-friendly content without having to involve a programmer.
If you are using the systems we’ve developed and shipped with the engine, you can lean on BPs for the vast majority of your needs. However, those systems are meant to be extended in C++, so customizing the systems or creating your own would need you to work in C++
A great resource for learning how this works is the Lyra project, or the Action RPG sample project. Both use C++ and BP.
Also, check out this presentation by Chris Murphy on how to start writing C++ functions to be used by designers: C++ for Blueprinters - Unreal Engine
–Tom