I think the issue with Parker is that his main talent is scoring, and scoring is one of those attributes that doesn't always translate from college to the NBA. Many times in the past there have been elite college scorers who's scoring ability never really translated once that player was suddenly playing against guys with NBA size athleticism and talent.
Rebounding and defense however tend to translate better. Guys with elite defensive talent In colledge do tend to become elite (or at least very good) NBA defenders.
That's why I think Embiid, Wiggins, Smart and Exum are lower risk players in a way, because even if they do not reach their offensive potential they are likely to be (at the very least) excellent defensive players. On the other hand, if Parker's offensive dominance doesn't translate in to his NBA game he will be a nobody...aside from a solid rebounding SF, which on its own has minimal value in this league.
Also the league today highly values athleticism when determining upside. A young rookie with the type of physical gifts that Exum, Wiggins and Embiid have will have super-high trade value on potential alone, even if they don't work out in their rookie seasons.
I actually think the safest pick is probably Smart. He had the right attributes mentally (IQ, high motor, desire to win), he can play either other guard position, and he is a strong performer on both ends of the court. Doesn't really have a huge hole in his game, and he'd likely to he a very solid starter at the very least.
Personally though, I'd still take the potential of Exum/Wiggins/Embiid over the 'safety' of Smart.