Here are a few things my game does that I couldn’t use blueprints for:
I override AGameMode::InitNewPlayer to pull command line parameters into UE4.
I added a Custom Movement Mode for climbing that queues up moves into UE4’s predictive movement system (super important for smooth network play and also enables the network replay functionality).
I added an expression parser to turn a math formula from a string into an expression that can be evaluated.
There have been other things in the past where I had to use C++, but then later those functions were added to blueprints, such as showing or hiding the mouse cursor and stopping an AI tree.