Physically, Bob is very hard to injure as the Sentry. He can take powerful hits from powerful people but nothing really keeps him down long, aside from when Thor kills him haha... his worst physical injuries were probably during his fight with the Hulk during WWH. When he was younger, pre-Sentry, his body was a lot unhealthier due to his drug addiction.

His mind is his biggest weakness. Bob has schizotypal personality disorder, agoraphobia, avoidant personality disorder, and DID, all of which leave him as a very nervous, unstable man on his worse days. I... think that's everything, also he just gets addicted to things easily; drugs, alcohol, power. His DID takes physical manifestation as the Void, a violent and usually unstoppable (except by him) force of evil, and most of his schizotypal-esque thoughts are related to the Void, some accurately and some not.