Dier still (when fit) does what Dier is supposed to do - be the other one in the midfield with a Dembele type. Sissoko doesn't do that, as the stats I've posted show. Neither does he do the Dembele role.
He defends a bit, he presses very little, he intercepts very little, he dribbles a lot but with little to no deep progressions, etc.
He can't perform the Dembele role our system is so heavily based around, neither can he play as the other pivot the Dembele role requires. Probably be really useful to a Stoke or a West Ham though.