How you throw, how hard you throw, and how often you throw are the main contributors to arm injury. Throw in some genetic predisposition and you have a very complicated issue.
We can't really point at any one factor and call it the smoking gun. Some guys throw with "great" mechanics and still have arm issues. Some guys break down at half the innings other guys throw without issue. Some guys throw with mechanics that make you cringe - but never have any problems.
As for a slider - yea, I wouldn't want a younger kid throwing that. It's the only pitch that makes my elbow hurt when I throw it - so that's enough anecdotal evidence for me.
I had a conversation one time in the stands of a 13U travel game. The dad told me straight faced that he didn't want his son to hurt his elbow throwing a curve - so he taught him a slider instead.