GDScript is not very maintainable as the code base grows. It lacks proper refactoring tools (e.g. the ones from Jetbrains Rider), static type checking, flexible object system and many 3rd party libraries which might be needed
I won't even get into how big of projects I've written in GDScript successfully.
I don't want to have any sort of undefined behavior
I want to have quick code reload button in the editor
I want to still rely of the engine official documentation with examples like it is with GDScript and C#
The purpose of recommending c++ here is:
If GDScript is too slow, reach directly for C++.
I'm specifically recommending GDScript over C# for ease of use and c++ over C# for performance.