The psoas muscle, in my experience, can be a problem, but it is NOT the only source of low back pain or abdominal pain.
There are a number of organs that lie in front (anterior) of your psoas that can make it hard to get to. Many of these structures can also lead to low back and abdominal pain. There are many feet of small intestines, the large intestines, your aorta and other blood vessels, nerves, kidneys, ureters, etc.
Check out my video interview with Allison Porter, MD, a general surgeon, 2011 crossfit games competitor, and former miss Washington, regarding rolling out the psoas. I am truly humbled by her knowledge and willingness to share with us. Thanks again Dr. Porter!
As a side note, I do find instances where I manually "treat the psoas", although I have long suspected it is more likely I am influencing other structures besides just the psoas (if at all), no matter what I think I feel. And this is, of course, after careful diagnosis to make sure the psoas is actually the thing involved (instead of: a kidney stone, a herniated disc, a tumor, appendicitis, a hernia, an abdominal aortic aneurysm, constipation, irritable bowel syndrome, inflammatory bowel disease, a cyst, ankylosing spondylitis, infection, etc).