However RAIDZ2 can survive up to one of those stressed disks failing, while a 2-mirror has everything riding on that single survivor. It seems intuitive that mirrors would be safer, yeah, but if you run the binomial distribution numbers there are some realistic combinations of array size and individual drive failure probabilities where RAIDZ2 really does appear to be safer

Imagine a gambler telling you "you can either throw one k-faced die and you lose if it comes up on 1, or throw n k-faced dice and you lose if two come up on 1". Depending on k and n, the second really can be the better choice

Often people cite the faster rebuild of mirrors as a safety advantage, but the same amount of rebuild IO will occur regardless of how long it takes. Yes, there will be more non-rebuild IO in a larger time window, but unless that routine load was causing disks to fail weekly then I doubt it will change the numbers non-negligibly. It will of course affect array performance though, so mirrors for performance is a good argument