These are established tech, so I would trust any model sold widely in Europe – the regulations being much tighter than for China exports

I myself could use a cheap one that is widely used in China; it's only the risk of getting a crap one off eBay/Banggood/others that I'd like to avoid.
(As an example, the cheap USB (12 Mbit/s) isolators based on ADuM4160/3160 are absolutely fine, and work just as well as "official" ones, because they are straight off the ADuM datasheet. The only component to check is the isolated DC-DC 5V-5V converter on it, to check it isn't too noisy, and how much current it can provide from upstream 500mA at 5V. If your USB gadget needs less than 250mA or so, it's not a problem. The Olimex one has a power supply connector for device power, that is isolated from the computer ground, and is therefore a good buy for power-hungry USB devices. But the cheap Chinese isolators work for normal USB gadgets absolutely fine – and is something I'd suggest using if one connects e.g. their 3D printer to their computer via USB.)
Here's some examples, no affiliation:
- CarTFT.com in Germany. They also sell bundles with known-good power bricks for 240VAC mains.
- Mini-ITX.com in UK.
- Mini-Box.com offices are in the USA, but they have a sales office all over, including in South Africa (included just because your location flag is Zambia)
I forgot to mention that they are very common in automotive uses (car computers); the -WI or Wide Input models are common there, because they can typically run off the battery voltage (which isn't nowhere near a stable 12 V).
Since it is just a set of DC-DC converters, I would not call the copies "counterfeits" either. So, if you get a cheap copy, it is quite possible it works just fine. The way I'd check is to first check the parts. Then, if you have an electronic load, I'd test the 3.3V and 5V rails at up to 8A (separately) for fifteen minutes or longer, to see if the board gets hot, or there is too much noise on the rails.
I've used the TI Webench to get examples of high-efficiency 5V-to-3.3V DC-DC converters, and I just now checked the 12V-to-3.3V suggestions. The LM25117 and LM5117 designs have a BOM cost of $12.71, require a board size of 1374mm
2 (say, 60mm × 22mm), and are 96% efficient above 4A (88% at 1A). (At 8A each at a conservative 92% efficiency the 3.3V and 5V converters produce 2.3W and 3.5W of heat, respectively. At 96%, just 1.1W and 1.7W; less than three watts of heat at full tilt.) So, there is
nothing surprising in PicoPSUs, and one could probably design an equivalent oneself – except that the "official" ones have been used for well over a decade, and have had the benefit of practical testing and real-world fixes and enhancements.