the potential difficulty of the concepts necessary to formulate the solution

As I see it, there might be considerable difficulty of concepts in formulating even the exact problem statement. For instance, given that we want a 'friendly' AI; our problem statement very much depends on our notion of friendliness; hence the necessity of including psychology.

Going further, considering that SI aims to minimize AI risk, we need to be clear on which AI behavior is said to constitute a 'risk'. If I remember correctly, the AI in the movie "I-robot" inevitably concludes that killing the human race is the only way to save the planet. The definition of risk in such a scenario is a very delicate problem.