Yeah, there are exceptions to every rule, because drafting is very very difficult, and very hit or miss.
But there are overall trends that are valuable to keep in mind for "tiebreakers," or to influence overall strategy. And, in the long run, these end up being more helpful than not; they can't predict everything of course, and there will of course be exceptions, but we could pick exceptions all day long and get nowhere.
IN GENERAL, things to be concerned about include:
- Unimpressive early seasons with steady improvement to an "impressive" level. Usuallly this is just getting bigger and stronger relative to watered down / young competition, and not indicative of acquiring skills that will help stand out at the much more difficult NBA level. (An exception would be Brandon Roy. Steph Curry is NOT an exception. His freshman/sophomore seasons were GREAT, it just took some time for Davidson and Steph to get NOTICED which is different than being GOOD, which he always was).
- Older than others in your class. Just less room to grow, and less impressive if you are a 22 year old sophomore bashing on an 18 yr old freshman. If you are 18-19 and contributing in college, you are more likely to possess the natural skills to contribute at the NBA level when older
- Lack of athletic markers (steals, etc) in college. NBA selects for the elite, and you usually need to be above average athletically just to stick (people who are "average" athletically in NBA are just average relative to very good athletes.
So I would be cautious about Hield, but not surprised if he was Reddick or maybe even Roy level, and rank him accordingly. But he at glance appears much less impressive than, say, Harden, who shockingly had > 2 spg as a freshman and a great 3p% starting early.