The kit I used wasn't one of the "famous" ones. It looks to me from reading around and my own experiences now that any kit can be susceptible, especially when mixed with malt extract. From your logic that would make every kit "wrong" and no-one should use them.
Since this is the BEGINNERS forum and many beginners get into HB via kits, to blanketly say they have made a "mistake" and "you got the wrong extract" I think is wrong. You're putting the blame 100% onto the beginner in your reply, and I don't think that's where the blame lies.
Certainly for me, one I finish my current kits, I'll no longer be using kits - not for fermentation reliability but for taste.
Impossible in Japan. Very limited number of retailers, all selling the same products from the same batches. Best you can hope for is good storage.
You're the one who mentioned these so-called "famous" kits. You also mentioned "improper balance of sugars", I, personally, have never heard of a kit being famous for having unbalanced sugars. To be honest I don't even know wtf improper balance of sugars is supposed to mean. For that matter I don't know wtf "especially when mixed with malt extract" is supposed to mean either. I have never heard of a beer kit that did not contain malt extract.
OTOH, I have never once implied that kits are anything but the best way to start brewing. In fact, until you have lots of brewing experience kits are the best way to keep brewing too.
And you're thinking way too short term. The point is that if one buys a product that consistently is of poor quality, then the onus of blame is on that consumer for continuing to purchase that product.
In my experience I have purchased iffy quality extract from vendors and simply stopped buying it from them once I realized that the quality of their product could not be trusted.
All that said I think people put WAY too much emphasis on what the FG is. If I brew a beer and the FG turns out to be 020 then so be it. If it turns out 008 then so be it. There are 10 threads started here per week alarmingly stating that "my FG is 020" what should I do. The answer is "nothing". There is nothing wrong with a beer that finishes at 020 even if the expected fg was 014. If the beer is finished then bottle it and drink it when ready.