And cops use bullets, too. Soldiers use even better bullets, and these are a super effective method of disciplining people with great longterm success in reducing repeat offenders.
But the cost on the detainee (who is to be presumed innocent, who may be in distress themself, who may have a psychotic episode due to some unknown cross-reaction with medication or the like, who may be deeply scared) is much, much, much higher, which is why this sort of thing almost never happens where I live (not the US, obviously).
I feel there are good reasons to assume that force-injecting someone with a megadose of Ketamine without their knowing what they're getting, understanding what they're getting into, competent assistance during what follows, in a profoundly bad state of mind, likely in deep fear, under heavy stress, while being restrained (otherwise, how would you inject?), with no knowlegde of their medical history or current condition, administered by laypeople, who would be completely useless in case of complications – I believe that isn't quite the same as tasing someone, or using pepper spray. While it's not visible in the way beating injuries are, that sort of thing has big potential to really deeply traumatize and mentally scar people for a long time.
Cops here do not have such powers and no one has tried to give them such powers, so far; I hope it stays that way. If someone has to be sedated, a doctor and an ambulance have to be summoned, and the then-patient has to be brought directly to a clinic that can take care of them; all in a medical setting from that point, not primarily an arrest (though, of course, depending on the situation, they may not be allowed to just leave once they're lucid again). This is only ever to be done in case of people in a psychotic episode that cannot be calmed otherwise or the like, it's not something to put into annoying detainees to make them shut up and be easier to handle. Any emergency doctor willing to inject such drugs without proper medical indication would take on big personal risks.
And, finally:
> Things are a mess if we deny the need to impose proper behavior by force.
Things are a mess as well if the tools to do so are passed out without checks and safety mechanisms that are appropriate to their effects. Letting cops shoot people at will while there are other, less severe options, that's awfully bad. And letting them drug people into submission while there are other, less severe options, that too is really bad.