Yeah I felt that was very harsh to Belfort too. Stann has a bit of power behind him, but is pretty generic at striking in all forms. Belfort has timing, speed and sharpness. Belfort isn't the strongest ever while Stann has natural strength, but Belfort generates so much more power through punch technique that if he lands on Bisping, you can at least expect the same results that Stann had when he landed.
The biggest thing to me is Belfort's finishing ability. If he rocks you, he stops you. He's one of the best in the world at it, maybe only having Anderson Silva ahead of him. Bisping proved that his chin isn't great and that he is pretty open to getting tagged in the Stann fight, and assuming Belfort's striking is only equal to Stann's, if he rocks Bisping he's gonna finish it.