What are two obstacles to understanding To Kill a Mockingbird?

The obstacles that I think would be the most troublesome would be the reading level and the subject material of the book. The book covers racism at an extremely adult level, with the title leading you to believe that it's actually about a bird. The reading level requires a higher level than what is usually available to the average student. To get around this, you would need more discussion on the book than what is normally provided when this book is provided as a subject of learning material.

Culture difference and era. Both the culture of the people in the book and the setting are different from now, and thus makes it harder for students to understand.

Laws - they might not have experience in the courtroom

Prejudice in the old South - they might not understand the tensions/prejudices that existed post Reconstruction