Interesting topic. I don't have a firm opinion one way or another. My son wants to do the basic Driveline 8-week program starting in Dec leading up to HS tryouts at end of Jan., and I plan to support him and monitor him closely (says the guy who let him throw too much this past weekend). With Driveline, they seem to emphasize ensuring that the player is ready to start the program (age, conditioning, range of motion), coupled with proper warm-up, moderation, rest & recovery as key elements to the program - if properly followed.
To me, there's seemingly nothing magical about a 5 oz ball vs an "under-weighted" or "over-weighted" ball. It's all relative. The standard 5 oz ball is a circumstance of history, not a carefully considered weight for a particular age group based on bio-mechanical studies over long periods of time and large player sample sizes. The way I see it, to a 12 yr old (for example), a 5 oz ball would feel considerably heavier (and place considerably more stress on developing bones, and arm/shoulder tendons/ligaments) than the same ball would to a typical 18 yr old. Yet I don't hear anyone advocating that pre-HS kids should be avoiding 5 oz baseballs.
Again, I don't purport to be an expert in the field - far from it as I'm just a dad whose son is a 2019 HS ballplayer. But it's unclear whether there's enough evidence to say emphatically and conclusively that weighed ball programs are the end all and be all to increasing velo, or that weighted ball programs are all bad and should be avoided at all costs. Like anything that involves increasing the limits of the human body, moderation, nutrition, rest & recovery, and paying attention to one's body's own feedback seemingly would reduce risks (though not eliminate them) significantly.