Obviously it's great to have a player with size.
The problem is when the MAIN focus and purpose behind drafting a player, especially with a high pick, is size. With a 1st rounder, you better be getting a guy who has some combination of well above average, if not elite, skating, hockey IQ, shooting, passing, puck carrying, etc. If you take a guy who has size, and then is mediocre to slightly above average in those other areas, it's seems to rarely turn out.
Also, these guys frequently dominate junior simply because they weigh ~30-50lbs more than everyone else, and their skating/other skills is passable enough for those leagues. That advantage is largely negated by the time they reach the NHL, and then they're well behind the curve in the other areas relative to NHLers.