Stiftung Warentest is definitely useful, but like you said have a spotty track-record for tech products. Realising that definitely made me more sceptical of them. It's basically the concept of "Gell-Mann Amnesia" - if I can tell their tests are flawed in a field I'm knowledgable in, are the tests they do in other fields that seem reasonable to me also flawed (but I ignore that)?

Although they do come up with the testing protocol, I'm pretty sure they don't do the testing themselves, and contract it out to specialist labs/testing companies (not a bad thing).

In general though, I agree it's easier to trust them, being an independent foundation, and getting no commission. Wirecutter got acquired in 2016, too.

I've also found it hard to get that German experience in other places. At the core, it's just down to how much/how many people care about something like quality. I think the (German) federal regulations and Stiftung Warentest are a result of this culture, but also make it easier to care about quality. It's a nice feedback cycle. The internet can substitute, but it's much more effort.