I've been routinely grilled and extensively searched many times just crossing between the U.S. and Canada - the most hilarious instance being when I was crossing from Yukon Territory back to Alaska some 10 years ago.
In Latin America, things are considerably more professional, and the government officials don't act like they hate you and just want you to go away. Europe's fine too.
I was traveling as part of a party of five (different travel dates, separate ETA filings, no apparent connection). One of us didn't get an ETA. Another got one just before the trip (and it was a 13-year-old child). The rest of us got ours immediately, but I was questioned at the border for 10+ minutes with no apparent reason, and then allowed in without a comment.
Depends. Someone I know had a terrible experience crossing into Belize on a land border. Was held there for two hours while they "verified" something. Seemed like a power-tripping agent.
I agree with you that US citizens aren't supposed to be fingerprinted on entry, but I am absolutely unsurprised that it happens sometimes.
Yay for our AI overlords or something?