There are two sides to signing an NFL veteran with a proven track record. They could be brilliant in providing instant quality and leadership to a roster. Or they could fall off a cliff and lose motivation like Haynesworth when he joined Washington in 2009 (via The Athletic).
He was excellent in Tenessee with two All-Pro selections before he joined the Redskins as a free agent. They handed him a $100 million, seven-year deal. He instantly showed up overweight and unmotivated. His signing was a complete disaster and a warning to every NFL franchise.