you intentionally used Marvin's as example of someone who didn't translate well to the NBA, using this Marvin case to solidify your claim that Barnes will "also" won't translate to the NBA, this is a classical case where players comparison is used to question a prospect's game.
Any prospects will be written roughly similar in terms of shooting as their strength, especially someone with a good follow through jump shot with great form, its obvious scouting will report similar quoting, even Beal was mentioned on his jumper in this instance from draftexpress:
there's very little not to like about his shooting form, as he gets great elevation, possesses a consistent release point, shows excellent follow-through
Right now you're putting Marvin's weaknesses into Barnes comparison? Let me show you what you need to look at, in an objective way,
draftexpress on Marvin
Offensively, [Player] has only shown that he can score in one of four ways: open stand still jumpshots, lay-ups / dunks on fast breaks, put-backs and free throws.
draftexpress on Barnes
Barnes is an outstanding shot-maker, ...he should end up being a versatile scoring option who can catch the ball in different areas of the floor, whether it's posting up smaller players, catching the ball in the mid-range, running off screens, or being used in isolations and pick-and-rolls on the perimeter as he improves his ball-handling.
draftexpress on Marvin
[Player] hasn’t shown he has the ability to put the ball on the floor and create his own shot consistently, whether it’s driving around a defender to get all the way to the rim or merely shooting off the dribble. If [player] is going to be a great small forward in the NBA he will need to develop this part of his game.
draftexpress on Barnes
Barnes' biggest strengths revolve around his excellent scoring instincts and offensive polish, effectively utilizing shot-fakes and displaying advanced footwork when catching the ball on the perimeter and in the mid-range area, which he uses along with his great size and frame, to create space to free himself for jumpers.
draftexpress on Marvin
Also, [player] has shown no post moves outside of trying to overpower his opponents with a drop-step toward the basket. When surrounded by defenders in the paint [player] often gets his shot blocked because he has yet to show a jump hook, a fadeaway or an up-and-under move.
draftexpress mentioned on "posting up smaller players" as one of Barnes' scoring option and didn't mention any negativity on it. Barnes rarely gets his shots blocked when posting up, and he does fadeaway too (whether its posting up or off the dribble)