Well, not being an expert, I can only relay what I know to be personally true: My 2016 son was/is a fairly hard throwing LHP. Jeff Holly was referred to us by another pitcher, so we went along to Irvine to see what it was all about. Jeff spent 15 minutes watching my son throw while making small talk with me, then went over and asked the boy a few questions about mechanics, stride, hips, hand separation and a few other items (I'm guessing those were the items he wanted to address after watching him throw). He made two corrections, asked a another couple questions, then made one more correction.
Since that day, my son has markedly less arm fatigue the day after a start and recovers faster than before. Jeff will tell you up-front: "My job is to make you the best 18-19 year old pitcher you can be, not the best 14U or whatever pitcher. Every pitcher is sore after a start. My job is to make sure you're sore in all the right places."
One more item of note: My son tried (and failed miserably) to throw a decent change-up for 3 years. After about 4 sessions with Jeff (working nothing but mechanics and the fastball), we brought up that issue. In literally 10 minutes, Jeff taught my son the change-up he uses today....and it's nasty.
Please don't interpret these statements as "If your son wants to pitch, Jeff can get him there.". If you're not a pitcher, nobody can get you there. Jeff Holly is definitely a great option for good pitchers to get to the next level, wherever that may be.
FYI: Jeff was drafted out of HS as a 1B/LHP (after hitting 54 career HS homeruns with the ancient 1970s aluminum bats). He knows as much about hitting as he does about pitching.