Shamar Stephens is a strong defensive tackle with a great frame for the position. Early in his career, there was a worry that he bogs himself down and finds it difficult to move, but he actually carries his weight well. For a player of his size to be slotted into primarily a pass-rushing role at Connecticut is impressive, and he can find ways into the backfield.
That said, his measureables aren't quite of the "freak" variety. With a 5.24 40, and more importantly a 1.83 second ten-yard split, he measures out to below average at his position before accounting for his weight. A Barnwell Speed Score of 79.4 is somewhat poor (Aaron Donald's was 119, but Louis Nix was 78.4) and it would be nice if he could help his case with a more consistent showing off the snap he'd go a round earlier or better in the draft.
He anchors well, and plugs holes when double-teamed, making him an occasional nightmare for zone-blocking teams trying to create multiple lanes, and making things a little easier for the linebackers in the run game.
Stephens is as strong as his body implies, and has both the leg strength (an astonishing vertical of 30.5", the third-highest at his position) and upper-body strength to push people around and engage the blocks. During his final year at Connecticut he kept disrupting the line of scrimmage and can move through offensive linemen at times.
He has a powerful punch that he's willing to use as a three-technique and knows how to attack the center of gravity of offensive linemen in order to make an impact. He's also played as a one-gap nose tackle and at times has taken on two-gap roles, controlling offensive linemen at the point of attack. He works through blocks, consistently drives his legs and has a high motor with intent to keep making the play even after getting stopped on the initial move.
His long arms (34.5") have allowed him to corral running backs and expand his range as a defender, but his limited pursuit capability, and his movement skills are more lateral than they are straight-line—meaning his ability to make plays after the play has moved in another direction is limited at best. At the next level, this will limit his pass rush production as well, and he's better served to be a nose tackle that holds his anchor and blocks.
He has very good balance and he can pair that with generally good pad level off the snap.
Unfortunately, I don't think he'll make the roster. As a nose-tackle only prospect that looks like he may be better as a 3-4 defensive end, his inability to great backfield production at the next level will limit him, even if he has experience holding the point of attack. the fact that he keeps working blockers doesn't mean he does it well, and he can't disengage to create space effectively.
That he has limited chase range is important, even for the bigger bodies on the line, and his slow reaction time may be what dooms his roster chances. Getting off the snap is important in the NFL, and it's something that Fred Evans holds over him with a significant advantage (and Chase Baker, too, to an extent).
Nevertheless, it's good to take a chance on a player that shows at least one ability (holding double teams) that the Vikings have missed for a long time. Perhaps he can clear the way for linebackers to come.