I don't think weighted balls cause injury but I do think the pitching s&c and development coaches do pat each other a little too much on the back. Everyone talks about how great pitching dev has become and how it is ahead of hitting and in some regards this is probably true but pitching dev has done absolutely nothing in the injury prevention department.
I mean there are tons of theories (get strong, do bands, do long toss, take time off, get flexible) but really nothing has worked yet.
Now I don't think it got worse than 20 years ago but no great progress either, injury rates still high.
Pitching dev has improved velo and pitch design but the next step absolutely must be a significant reduction of injury rates as this would be a huge competetive advantage.
Imo the pitching dev guys don't hold themselves accountable enough for injuries which are accepted as part of the process.
Don't get me wrong it is better to properly lift and condition yourself rather than getting fat in the off season like pitchers 30 years ago but regarding health the progress isn't great.