I deleted an earlier - less positive - post.
However, I think the point is still valid that pitching coaches - by definition - are there to analyze all aspects of a pitcher's ability - not to just glue on a few junk pitches.
You will hear almost as many opinions as people you visit. I am sure that Mr. Hillhouse will tell you that his way will be the best, and who I am to disagree. Most pitching coaches will do that.
Here's the point though. There are a few standards that I think by which you can judge all pitching coaches.
Do you like them and feel like they listen to you and hear your concerns? Are they able to give you rational and reasonable explanations of the things they will ask you to do (or tell you not to do yet?)
Over time, are you showing progress? Is the progress in the area that they were working on and that they predicted would get better?
Can they talk about what will happen over a period of time?
My DD is 13, and has had lessons for 4 1/2 years. She throws 4 pitches - fastball, change, drop, and curveball. We play with lots of other pitches, but her pitching coach makes it clear that the rest of those pitches come later. Some take years to master. But the later pitches (screwball, drop curve, rise curve, rise ball, knuckleball) build on the basic pitches she continues to master. This makes sense to me.
You might not be able to throw a good knuckleball unless you have a good _______________. (Fill in the blank, pitching coaches!)
So try different people, but it is the consistent following of one philosophy that will show improvement over time, in my opinion. If the progress stops, or if it stops making sense, it is time to move on.