Yes you can. I have made a full AI system using Inheritance, Blackboards and behavior trees. I have more than 6-7 types of AI and 3 different bosses in the game. I have implemented a whole system and a tool for my designers to even level design, all in blueprints (We did not have time to learn Unreal C++ as we had to make this in 3 months). However, I did feel a lot of times I had more time and moved all the code to C++, that would make blueprints cleaner.
In short, you can get almost anything in Blueprints, if needed you can also write some classes in C++ and use them in blueprints as Blueprint function libraries, components, Interfaces etc.