I don't know about police, but the most unpleasant experience I've ever had at any immigration was in Japan.

They were NOT polite. Even if they found nothing wrong.

US immigration is a day at the spa in comparison. Even when the consulate messed up my fingerprints. Cleared up in under two hours, they were cold but polite the entire time - not angry and disrespectful.

After I was finally allowed in Japan, 4 hours after I had arrived - thanks to my uncle, a Japanese citizen - everything went fine. Nice people, spent three months there.

I'd like to visit again, but I'm dreading the immigration experience.