The picture looks as if someone had trouble with the trigger guard installation and either weakened the area or just cracked it (dpms example )
I have owned a few dpms lowers and never had a problem.
I am not just hearing what i want to but giving first hand knowledge to why i wouldnt hesitate buy one ....
I work with metal of all kinds on a daily basis and can reference many cases where the folks who made the metal are at fault for problems that occur in many different processes welding,machining,bending and alot of times the problem wont show its face until after the product has reached the field or process it was built for.