Though Fury knows how to box in a southpaw stance, he seems absolutely clueless how to box AGAINST southpaws. Once Ngannou changed his stance to southpaw, Fury was troubled by it and just couldn't land even when he tried. He had big problems and a bad cut against good, but unexperienced and light-hitting southpaw in Otto Wallin (and that was better version of Fury). Once he faces an elite southpaw in Usyk who uses deifferent feints and angles, he'd look helpless. Also Zhang with his counterpunching skills and great handspeed can put Fury on the floor, and Zhang is bigger and hits harder than Ngannou.
Yes, he ain't all that against lefties, but to be fair he was nothing against Francis. A cocaine shade of a man.