LHPMom -- I do not want to speak for Coach House, but have had this discussion with him a couple of times. This is the understanding I walked away from those conversations with:
All pitches produce wear & tear. It's called physics. The "safest" breaking pitch for 18U Pitchers is the full Karate chop Curveball. It is the easiest position for them to pre-set and hold throughout the delivery, without twsiting the arm during delivery (a big no-no).
At the risk of inviting the pronation guru's into a conversation not intended to go down that path ... yes, the natural throwing movement ends in the arm pronating (plam out). When you throw a FB, the palm goes from flat, to pronated out. when you throw a CB, the palm goes from pronated in, through delivery, to pronating out after release. Yes -- there is "twisting", but it is happening in the post stress, deceleration period.
Yes, the Screwball starts pronated, but most kids don't have the strength to hold it; adding more strain to the delivery. I like the CB for 14U to 18U players because they can hold the position through delivery without strain or twisting, and the velocity change (-10 to -12) is great for getting out HS hitters. If you can spot a fastball, and throw a CB or CU for strikes, you can get through most HS line-ups.
For what it's worth.