Great answer by @Datawrangler above.
SkookumScript is much simpler than C++. Certain types of tasks are much easier to create in SkookumScript than C++ - particularly high-level tasks. This includes essentially the gameplay or stage-direction aspect of game development: AI, game mechanics, stories, missions, automation, etc. Concurrency (the interaction of several things operating in the same time frame) is several orders of magnitude easier in SkookumScript and there are other game concepts built right into the language.
SkookumScript also has the SkookumIDE and custom UE4 editor and runtime plugins which facilitate realtime commands and modifications. This can hugely positively impact your iteration speed and workflow.
SkookumScript is simple to use because it is optimized for high-level use. It is in no way dumbed down so that it is less powerful at this level.
However, C++ is better at many lower-level tasks such as number crunching and pushing out polygons in rendering systems. SkookumScript itself is written in C++ and we are big fans of C++. Fortunately most of the C++ for low-level systems that most people need is already written in the Unreal Engine and the SkookumScript internals.
Both SkookumScript, C++ and UE4 Blueprints are all great tools. They are each better and worse at different things just like a hammer and a chisel are better and worse for different carpentry tasks. It is less a question of functionality and more the trick of knowing which tool is best for a given job.
At the same time, SkookumScript is designed to work in tandem with these other tools so that you can pick the best tool for a given job. So when it makes sense to write some low-level C++ code it is easy to call it from SkookumScript and for it to call SkookumScript. Same goes with SkookumScript and UE4 Blueprints.
The biggest current limitation of SkookumScript is that it is light on documentation. Some docs, tutorials and examples exist though nowhere near to the level that would be best.
That being said, we know of several people that have picked up SkookumScript within a couple of weeks with no previous coding experience. People with existing coding experience can usually pick it up in a day or three though they may find SkookumScript unique/unusual in some areas.
Even if you aren't using SkookumScript to make the core of your gameplay from day one, I recommend that you use the SkookumIDE and its console and its workspace (also known as a REPL - read, evaluate, print loop) to run commands live while your project is running. This is a benefit from day one and will help you pick it up at your own pace. You can then add in bits and pieces of SkookumScript gameplay as your mastery grows.
Good luck and post any questions you may have here in this forum and we'll do what we can to help.