Well, there is
one report where a SD card died after attempting to overclock, and that report looks legit to me (OK, it's just one case from maybe some hundreds of testers). There's also my old CF card, which died after an incorrect software request, I think (an overflow in my testing code, but I'm not 100% sure - IIRC overclocking wasn't even active when that happened). What I want to say is that card-related experiments are not exactly risk-free (OK, nothing is, but this area in particular is a bit more problematic), and the users should be aware of that, especially if testing on expensive cards.
Thing is, we don't really know what happens electrically (whether the voltages are correct for the UHS modes, whether the timings are correct or just on the edge, i.e. happening to work with most cards, but not with others etc - one would have to probe the SD signals with an oscilloscope and check them against the specs, which I don't think any of us did). Also, we don't know exactly what the low-level registers are doing - we only know that some values work at least on some cards, with different levels of performance and reliability, and others values don't work at all. Whatever is behind those values, is still a mystery (of course, there are some educated guesses about what each register might do, but that's pretty much it).
Another anecdote, not related to ML, but from another experiment I did: a 5V sensor connected via UART to a 3.3V microcontroller. Apparently fine at first sight, so I didn't think too much about it. Worked fine for a few days of continuous use, then it stopped - the MCU pin that was receiving data from the sensor, appears to be toast. So, you can't really say there is little to no risk, even after several hours of continuous testing, when there is a pretty clear report of damaged hardware (in my opinion), even if all other tests were apparently safe. That scenario might or might not happen again - we simply can't tell.
Not saying users should not run the tests, but - in my opinion - they should be aware of the small chances of damaging the SD card hardware. OK, there is the "you get to keep both pieces" thingie, but that doesn't mean we should rely on it. OP asked specifically what are the risks for this particular test, so...
Food for thought:
https://marcan.st/2011/01/safe-hacking/