I have always, since i was young, not trusted bought extension cables to be anywhere close to their purported rating. I make my own in most cases, using industrial socket outlets, cable glands (I have a mix of metal and plastic, but tend to use metal ones now where possible, as they are much more robust), proper SABS cable for high power, and decent plug tops. Getting hard to find decent plug tops, even those that are sold by electrical wholesalers are often now hollow pin, or are pressed plate, with a steel ground pin. Cables are getting thinner, even those with SABS markings, made by Aberdare cable, are being made with the copper cores on the absolute minimum diameter, and with the plastic also being lower quality, as 30 year old cable is noticeably thicker, has more copper in it, and is still more flexible than the new cables.
Sockets are using thinner brass, poorer quality switches, which is why I salvage old ones, take them apart as they are screwed together internally, and retension the contacts, clean and lubricate the switch, and put them back in service. Same for regular socket outlets, I treat them as only being rated for 10A, and even then you will have it fail after 3 years or less, because the switch, even if not used, will fail, either falling apart, or burning up internally. Most common thing to replace in any house, especially those that are used often, like in a kitchen, or those with a large load plugged in, like a microwave oven or fridge, which both have high start current peaks. Others just fall apart, and these are not the no name house brand ones, but the ones with a brand on them, they all have outsourced production to the same small group of Chinese OEM's, and they all use thin brass, even thinner internal straps, and rely on the brass to provide tension on the plug pin, which means you get poor contact, not helped by the contact area being very small, often only an edge of a sheet. not like the old ones, thick brass formed socket, with a steel spring providing consistent clamping pressure on the pin, and with a large area of brass wiping the contact surface clean each time, so you get good contact. Use a thermal camera and you can see that the cheap socket heats up a lot more than the old quality ones, even with a load of only a TV set on it, so 100VA or so of load, which should not be ocurring, but is common now.
Basically stay away from hardware store brands, even though they are convenient and open on weekends and late, and buy from reputable electrical wholesalers, the big ones that sell to municipalities, big industries and you get slightly better quality, but will pay more. Avoid the malls, especially those with nothing with SABS approved products, as those are all made as cheap as possible, and often you find cable made from steel wire, aluminium core and mystery metal, which is a few strands of something possibly containing copper, but it is brittle, pokes through the insulation, and you have 3 or 4 strands in the cable, and a very good chance of it getting hotter than the heater you plug into it.
so that one isn't even an issue.