Volleyballs work, lighter with a little resistance, maybe I'm wrong, but sometimes we use the big pads from the football team to beat into, kinda like a heavy bag.
Different strokes for different folks. They do these types of things in the Latin America and last time I checked they where producing better major leaguers.
Warning this post may be long or windy!
Try to keep in perspective most of these kids we are talking about are females and do not have the forearm strength or wrist strength the potential major league player has or will have.
We hit a weighted ball that is about 10.8 ounces and others that are 11.8 ounces. I am not exactly sure however I think a regular softball weighs about 6.75 ounces so it provides resistance verses coming to a complete stop which is were the stress on the elbow, shoulder and wrists take a beating. Practice smarter not dangerously with a sound reason or logic.
Without naming the team, every girl on the team was having shoulder issues and I jokingly said what are you doing hitting basketballs and both parents who happened to be coaches said yes!
We played against team Venezuela and their coaches do not use these drills and they were good hitters and are fundamentally sound they need pitching however.
Look at the differences in male/ female ACL injuries by just going to Google and type in female ACL injuries...it is an education and starts from the ground up and being in a strong balance athletic position so all drills are not created or done equally.
The female differences would be a good thread...