The kid was drafted in the 20s for a reason. He has an under the basket game with limited physical quickness upside and he's a slow jumper. These are all problems.
On the other hand he has a massive frame for such a young kid, good form on free throws and he keeps his head up both on offense and defense. He's moving, he's looking and a guy like that can go a long way in the league. If he adds some muscle to that huge frame he's going to be a tough matchup against the smaller big men in the league. Of course, he's a kid and has plenty of time to learn.
But here's why I think he may succeed. He plays with a little nasty. Go look at those videos. He's setting hard picks. He's shoving guys out of the way going for the rebounds. He's a big strong guy and he's not afraid to use it. Oh, in the NBA he'll be a foul machine for a couple of years until he gets better at hiding it but this guy has the attitude to succeed. He's not likely to be intimidated or back down. I like players with a little nasty. It gives them an edge. Zizic has that and thus I think he'll be a valuable contributor down the road.