I don't think it's specific to SK, because it works either way(C++ or BP project), but I personally start a C++ project(because I know I'll usually write at least a little anyways). SK is an engine level plugin so it's already present when you start a new project regardless of the type(if the plugin is installed).
So soon as I start a new project and open it, the SK IDE and workspace opens right alongside it automatically.
SK to me particularly excels in situations where you have concurrency and a need of coroutines. Having easy to use out of the box support for these concepts makes a lot of complex AI or world interaction mechanics extremely easy to code compared to C++, where you'd either need to write your own library, use a third party one, or painfully alter things to keep in lock step with the frame count otherwise. SK really makes long-running processes that happen in the world really easy to reason about.
BP to me is really good for "one offs", I personally don't want to code complex systems in BP, but I think BP excels at simple tasks or actor/object specific scripts in the scene/map. It's often faster to slap a simple on_overlap BP onto actors than to code the equivalent in C++.
The other thing BP excels at is engine access, BP is the primary language of UE4 even if they don't market it that way. Many systems have BP support before C++. Like if you're touching the animation and matinee systems you want to use BP because using them via C++ is just a pita even if possible.
Those are really the major areas I use BPs in.