That's not the case it mirrored vdevs. There is no degredatuon of the array with a failed drive in a mirrored vdev, it continues humming along perfectly fine.

Also resilvers are not as intensive when rebuilding a mirror as you are just copying from one disk in the vdev to the other, not all X other drives and recalculating parity at the same time. This means less reads across the entire array and much much quicker resilver times, thus less window for drive failure.

But don't just take my word for it. This is a blog post that go much into much more detail https://jrs-s.net/2015/02/06/zfs-you-should-use-mirror-vdevs...