That is true, but chain promotions happen throughout baseball, there are players coming up to Greenville from short season A. But I think consistency is a hard thing to get from a pitching prospect who is 1 year removed highschool baseball.
The minor leagues are littered with pitchers who don't have great stuff but they know how to pitch and they can get away with that at a lower level. I think of a guy like Roniel Raudes, or a Brian Johnson who pitched really well in the minors but scouts never saw more of a BOTR/Swing man type out of him. Guys who pitch well in the minors get exposed all the time when they move up the ranks.
You also have the opposite, you have guys who have all the tools but they can't put it all together. Maybe they can't repeat their mechanics, find their release point, or various other things but they are young enough that if they put it all together they can be absolutely dominate.
I'm not worried about Groome, even if he laid an egg all season, but when a guy is supposedly MLB ready I do think you're right to expect more consistency (otherwise I'd argue he's not even ready).
The thing about raw skill and tools is you can't really teach them. I mean, you teach a guy to pitch but you can't teach a guy to throw 99 MPH. Just like you can teach a guy to hit but you can't teach 425 foot HR. This is true in other sports too, think Basketballl; you can teach a guy who to shoot but you can't teach him to jump and dunk it. Either you have these raw abilities or not.
This is the mystery behind player development and the suspense between trying to figure out who will boom or bust. You can't predict how guys will develop because in essence we're trying to predict the future and know one can do that.
What we do know, is the guys who have raw tools put it together, we can generally know how good they can be.
A guy like Jason Groome could be a top of the rotation starter, but he could also be the next Trey Ball too. I'm not worried about Groome, I have faith that he will develop into an above average MLB pitcher but if he doesn't I will not be surprised either.