This answer is highly dependent on the reader. I would say one strength is the way that Upton Sinclair is brutally honest about the horrific work conditions that existed at the time the book is taking place. I remember reading the book. It's been two decades since I read it, and I still remember the feelings of revulsion I had while reading the book. The factory descriptions were so gross. I still remember how sad I felt as Ona's quality of life declined as the book progressed. You want so badly for things to turn around for her, but they just never do. The book does an amazing job of tapping into reader's emotions.
Related to the strength of the book's brutal honesty is a weakness of the book too. It's a dark and depressing book. It ends on a real downer and doesn't leave the reader with much hope, optimism, or possible solutions. Sure, that might be more realistic, but hopeless readers aren't usually readers that are inspired to action.