His in-breaking routes are real good. You can see him plant and gain separation. His out-breaking routes don't have the same sharpness, plus defenders tend to cheat toward the sideline against him. He doesn't have much of a double-move, but again, defenders usually play him not to get beat down the sideline. Because they do tend to watch him for the fly patterns, his out-breaking routes should be more effective than they are IMO.
Teams haven't had to play Dez with deep help as much because he and Dak haven't connected enough to make them pay for having a CB carry him up the field. There are two things Dallas could do about that: 1) Run more in-cuts and out-cuts and fewer flies so that when Dez does run one the DB might bite on stutter-step or 2) Have Dak and Dez figure out how to connect when he's got one-on-one coverage on the fly (by that, I mainly mean that Dak has to learn how to place the back-shoulder or fade where Dez can use his physical ability to go get it).