Being treated like a criminal. Long lines, employees providing conflicting information. Being yelled at for following the direction of the previous person. One time (after returning from Asia) I accidentally gave the DOB of my wife when they asked (she was with me) and the result from that was being escorted back and having to spend 15 minutes convincing them I simply made a mistake.
Houston was always the worst but Chicago is painful as well. It's not just the lines, it's the attitude of the people who act bothered by having to work.
The smoothest transit I had through Immigration and Customs happened this past holiday season when landing in Atlanta. The latino families in front of were all escorted for additional screening so the white male American got to walk through without even a question.
Queuing is expected. Having traveled to Mexico, half of the EU, and multiple Asian countries over the past 2 decades, coming back to the US is the worst immigration experience that I've had. That's why I say "not pleasant".
I will say crossing over into Canada, I've had mostly non pleasant experiences with them as well. I feel they put the only rude Canadians on the border as their first line of defense.