These are things you could easily find out for yourself by perusing their website and reading their scouting reports, which I definitely would recommend...you may find that they're more realistic about the kids in our system and the weaknesses, risks, etc. of each than you might think.
In Groome's case, the short answer would be that he is ranked so highly because of his very high ceiling - though obviously ceiling is not a guarantee of anything, and no one's claiming that it is. SoxProspects has him projected as a likely #2/3 starter with a ceiling of a #1 but a floor of an organizational player. I would note also that Groome was widely considered one of the top talents in the draft well before we selected him, and that national prospect lists like BA, BP, MLB.com, KLaw, Fangraphs, etc. have all ranked him in their top 100, if not top 50, so this is hardly a case of SoxProspects.com hyping him up.
For my part, he's so young and has pitched so few professional innings to date that I'm withholding my excitement until he's done a bit more, for the reasons that you mentioned...but that doesn't mean there aren't valid reasons for ranking him highly in a system that has very few high-ceiling types left.