It was his attitude to Brexit that caused many of the issues. A well communicated, consistent policy that involved a (democratic!) vote (let's not forget, in pure Greek democracy, every decision is supposed to be voted on by the people!) was fine, but Corbyn was entirely incompetent in delivering, or showing any energy, or capability. He was a massive let-down. Ditto the initial vote, where he couldn't bring himself to get behind his party's position, but nor was he able to show his own convictions.
Of course, the biggest criticisms of Corbyn and now Starmer appear to come from the left-leaning people, which I too find absurd. On this board, of the right leaning ones, I can only think of Steve who comes close to criticising Johnson!