I'll grant you that Sadeas could be charming when necessary and that Sadeas was aware of his true nature, but let's consider. Sadeas has a reputation of brutality he may have the charm, but I think when it comes down to it he would be too much the blunt instrument. The darker side of Amaram rarely showed. It took Kaladin's extreme reaction, Dalinar's trust, and test to reveal his true nature. Everyone is pretty aware that if given the chance Sadeas will tear out their throats. He is one who can be charming, but he is known to be a politician who is ready to win at all costs. Ialai was I think the true politician. The subtlety I think was hers. He is dangerous, but she was worse. Amaram was a soldier of great skill, but just that. For all appearances he was not playing their game of intrigue. When one does not know that you are playing the game they are disadvantaged.