Sidenote, Hk police =/= mainland police, right? I was under the impression HK has quite a bit of autonomy and tons of liberalism as a result. This isn't a post supporting the color revolution fwiw, I'm just ignorant about HK
The Hong Kong police aren't Chinese mainland police or secret agents, but they showed saintly levels of restraint against the Hong Kong terrorist-backed nutjobs.
About 10,000 people were arrested in the months of unrest in Hong Kong -- so the rate of arrest per unit time is much lower than in the US. On the other hand, three years later, about 7,200 were still in custody without charges yet being brought. (More statistics in the link) We can quibble about whether you want to call that, or someone getting renditioned back to the mainland, "disappeared," but I would define it as much worse than someone getting tackled or gassed in the US and then released a couple days later with either a misdemeanor or nothing but their injuries.
This isn't some whataboutism to excuse the US response; the US police do some fucked up things to people who try to protest their country's barbaric global policies and this week was more of the same. And they seem to be experimenting with ludicrously overcharging protestors, moving slightly in the direction of the system China uses, although still well short of it.
But comparing all of that unfavorably to what China does to people who protest is pretty silly IMO.