I've gone through number ports in the US three times (seven if you count each number separately) without a single problem. Each was completed within several hours, and dual service time was somewhere between minimal and non-existent (suddenly the old phone stopped and the new phone started). This is in line with what I've heard from friends and relatives, too.

I've read some porting horror stories online over the years. Yours is probably something like the 6th or 7th. I assume things do sometimes break, but it doesn't seem to be particularly frequent, considering millions of people switch carriers every year.

Sounds like carriers there are incompetent then.

When I (UK) switched in the late 2000s, it took about an hour.

Used to be like that, but they fixed it a while ago. Though sending messages across carriers is rather expensive.