OL and WR is where I find they miss the most, but in different ways. OL they miss on guys by under rating them. WR they over rate so many guys.
Both of those things are probably rooted in the fact that HS OL are not going against other D1 guys very often, and HS WRs rarely have D1 QB throwing to them while they go against D1 DBs.
You bring up good points. OL also have so many variables in terms of how their bodies change over 4 or 5 years. You can easily take a guy that is 6'5 220 in highschool that looks like a TE and turn him into a 6'5 290 LT within a couple years.
Many of the best tackles were projected as TEs. OL is where good coaching makes a tremendous difference on a how a kid pans out.
WRs can show out in high school if they are a good athlete...but it takes more than being a good athlete to make it at the next level. WRs can make nice highlight reels, but it is hard to tell what they will do against much better competition when they can't get off the line of scrimmage freely.