I am not sure if your comments are directed toward JH and based on personal experience/personal testimony in college, College Summer Wood bat leagues and MILB or you are speaking to the game as it is played at HS?
That can be a very different game at the college and Milb level, in my opinion.
The hitters that JH is pitching too are not quite as restricted as my reading of your post suggests you think they might be.
While I would agree that there is success with throwing the pitch that is "unobvious," I don't agree with the way you have applied that.
To me, at the level JH is competing, it means, for instance, being able to throw a change in a 2-0, 3-1 count, with runners on base and still being able to throw it for a strike and hold the runner. It means being able to pitch backwards in innings 4-5-6 from innings 1-2-3, for instance.
What often times makes pitchers successful, without consideration of GO vs FO percentage, when they get to college and Milb, is the ability to command 3 pitches and to throw them in any situation. It makes a huge difference if they are throwing a 2 seam and 4 seam fastball, with command of both along with a change and a breaking ball.
It also makes a difference if there is a sinking vs. tailing action.
Without ever having seen JH, but knowing he is a lefty
, his higher FO percentage suggests to me he has tailing on his FB and further suggests he is able to change speeds effectively so hitters are getting on their front foot and elevating his pitches.
Knowing JH pitched primarily in relief, the question that I would pose that would need to be considered is whether he would have similar results the second time through a line up when hitters have seen him one time, seen every pitch he throws, and begin to make adjustments.
In college and Milb, if a pitcher is so predictable that he is primarily using breaking balls to keep a "hitter off balance," he is usually going to get crushed whether he throws his fastball 90, 94 or 98, unless he can doing something to create later movement off his fastball between a 2 seam vs 4 seam.
Additionally, if a pitcher is throwing a curve ball in a curve ball situation, whether he keeps it low on the inside or outside part of the plate, good Milb and college hitters are not going to "hit weak grounders back to the mound or third."