Whats a objective evaluation of his evidence?

Iago is a master of manipulation. He really strings Othello along without any real evidence. Iago knows that toying with Othello is far more effective than simply pulling out the handkerchief. Iago makes up stories about Cassio taking in his sleep and that Desdemona gave him the precious handkerchief. This way Othello falls deeper into his own pit of despair suffering, in his own mind, the ultimate insult of being a cuckold.