Dunn's mechanics are pretty bad. Buzelis didn't shoot well in the GLeague, but his form and mechanics are pretty solid, which is why I believe more in him becoming a good shooter at the NBA level. Dunn is significantly older and will need an overhaul.
Because as good as he is defensively, he's just not playable in today's NBA right now. Depending on the cost, he might be worth a trade up to a late 1st/early 2nd, but I wouldn't gamble heavily on him developing a jumper. We've seen that go poorly with Michael Kidd-Gilchrist, Josh Jackson etc. And even when guys DO develop a reliable jumper, it's usually not the team that drafts them that reaps the benefits, but usually the next team that signs them after their rookie deal.