Personally, I favor hitting people with billy clubs, because it leaves evidence of what happened and it requires the police to actually take physical action against another human. Giving cops needles full of magic compliance juice seems less responsible to me.
The reason people were upset about Rodney King was because it was brutal and easy to understand. Injecting ketamine is also brutal but it's less visible. I cannot imagine anyone ever getting over the fact that they were restrained by authorities and injected into a k-hole.
They won't be visibly harmed in any way, but that will absolutely mess someone up for life.