> their reviews are complete black boxes

They are certainly not "complete black boxes". This page [0], for example, explains in detail their test procedure for vacuum cleaners.

Just an excerpt on durability testing (via deepl):

> Impact test (500 cycles): The hoovers drive over thresholds 10 000 times and bump into posts 1 000 times.

> Drop test: The suction nozzle must survive 1 200 drops from a height of 80 centimetres.

> Cable pull-out test: 6 000 pulls.

> Test of hose attachments: 40 000 swings with the connection nozzle clamped.

> Squeezing the pipes, hoses and auxiliary air adjusters: The testers load them with about 70 kilograms for ten seconds.

[0]: https://www.test.de/Staubsauger-im-Test-1838262-1838266/