When Robert got drafted by the Raiders, they were a team in decline. A good player can sometimes get stuck on an underwhelming team and it affects his career. That said, he lasted eight years and made his money. Nothing to be ashamed of.
Imagine if he'd gone to a good team that used him correctly. Instead of a team in the Raiders who kept changing his OL coach just about every year. He also had 4 head coaches in his 7 seasons in Oakland.