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.