In most stores, self checkout customers are policed by the system to make sure that each item is placed onto a scale that weighs everything, and stops the process if weights don’t match up.

I’ve never seen that, and I’m not aware of any supermarket chain in my country that does this.

I've never been to a grocery store where the self checkout doesn't weigh everything. That's why people keep getting the "unexpected item in bagging area" error that requires an employee to come over to check and clear the error each time. This is to try to prevent theft. If you have more items than will fit into one bag, you have to periodically remove that bag and start a new bag. If you bump something or move things around while you bag (there's very little room to work with), you often get one of these errors.

Besides, if you’re planning to get a lot of items you scan while shopping, not at checkout. You get a portable scanner, put it slot on your cart and just scan each item as you put it in your cart.

I've never been in a store that has this. What stores in what country are you referring to? The anti-theft equipment for a system like this that would prevent someone stealing by simply not scanning something is probably a lot more expensive than the usual self checkouts. It probably has to use RFID or something and be able to effectively compare all items you're walking out with to what all was in the transaction. Do you exit the store through a specific gate that scans stuff or what?

Anyway, I think most of the people who are raving about how great self-checkout is are those who only buy a handful of items at a time, probably not stocking up on groceries or buying enough for a family.

If the store is busy I never try to self checkout since there are lines at all of them, people with full carts and the lines move very slowly compared to the ones with a cashier, where for the same length of line, my wait time is much shorter and then someone who's better at it than me, with a conveyor belt and ability to scan quickly does it, and there is usually also another person bagging, or if not I can bag as they scan (depending on the store).