It is appropriate to lump all prospects into the iffy category because all prospects ARE iffy. There is no such thing as a sure thing, and even prospects that were rated the best in baseball have failed.
I did not say that all prospects are equal. Obviously, some have a better chance of reaching their ceiling than others. But all are "iffy".
In a given year, I'd agree that the #1 prospect might not necessarily be a "sure thing." Don't know if Brian Buxton is a sure thing. Ben Grieve, Todd Van Poppel, Delmon Young. All # 1 overall at one time.
But, some prospects, as a practical matter, are sure things when on cusp of majors. By then, can see a lot. Bryce Harper, ARod, Mauer, Strawberry. Bryant was basically a sure thing to be at least a solid regular, not a failure.
Only thing really "iffy" about Bryant was whether more than solid regular and, if so, by how much. Never anything iffy about Harper, other than whether he would reach current level (and maybe beyond).
Never sure if player will reach upside. If we define failure as falling significantly short of upside, would agree no sure things.