As dedelman says, older draftees have a little less headroom to grow. Depending on your taste, however, you can see them as a little more predictable. Buchter is a pretty good 18 yr old, so I feel like I can't reliably predict where he'll end up.
But speaking in generalities... my opinion is that all scouting is fuzzy, even 20 million. Projections overshoot, I'd say, at least 80% of the time. In my mind the question is always "how much?" Anytime a 20 million projection says a rating is going to improve 30 points, I think it's too high. Experience says that's rare. So I ask what if it's only 20? Give a player five seasons in the minors, say coaching is a factor, playing time, possible long term injury. You do your best to manage all those things, but what if those ratings only end up increasing by 20?
Buchter's a good all round player, maybe a great one, but what if there's nothing special about him? He's not going to be a great glove at SS. Is he going to be a great hitter at SS to make up for the glove? Or is he going to be a 3B who has up and down seasons? Either of those outcomes are in play for him.
Also, his Durability is pretty low. For an 18 yo that's not unusual, it's probably the one type of player I believe Durability increases by a good amount for. But he's still only going to end up around 70 durability which is alright for a 1B but not so much for a guy you're going to count on at a defensive position.