This happened on a recent trip for us. We connected in San Francisco (to Kona, HI). Met up with my girlfriend's sister just fine. Her other sister and brother-in-law, however...

We flew from Toronto to SF, had a 7 hour layover. It was grand. We even went downtown and back with plenty of time. Her one sister flew in from Vancouver—also no problems.

Her other relatives flew in from Victoria, and got stuck in customs at SFO for over 3 hours when their layover was scheduled to be just over an hour. They had to rely on the graciousness of the airline to get them on another flight. They had to take a 4 am flight to Honolulu which had a 4-5 hour layover then connect to Kona. The Iron Man race was the next day that we went to see. They didn't get to sleep for 30+ hours. I mean, there are worse cases where you'll have to endure that kind of stuff. Calfornia->Hawaii is a pretty good place for it to happen. But still.

We were through US customs at YYZ in about 20 minutes.

On another hand we took the Victoria -> Seattle Clipper ferry and US Customs in the Seattle port just sort of hand-waved us through.