I was a Police Officer for 32 years and never had but one dealing with them and it did not show but I do not disagree with you. Just before retiring I was questioned by two ex agents and they were good and almost right but they did lie.
I would advise anyone to have a witness anytime they talked to an agent just for self protection. I learned a few years back that they would not record you,one would ask questions and one would take notes. This way they manipulate what is said.I always respected them but should have been smarter.
I hope lots and lots of people here and their loved ones learn from your informative post.