From
http://dtrace.org/blogs/ahl/2016/06/19/apfs-part5/ :
"Explicitly not checksumming user data is a little more interesting. The APFS engineers I talked to cited strong ECC protection within Apple storage devices. Both flash SSDs and magnetic media HDDs use redundant data to detect and correct errors. The engineers contend that Apple devices basically don’t return bogus data. NAND uses extra data, e.g. 128 bytes per 4KB page, so that errors can be corrected and detected. (For reference, ZFS uses a fixed size 32 byte checksum for blocks ranging from 512 bytes to megabytes. That’s small by comparison, but bear in mind that the SSD’s ECC is required for the expected analog variances within the media.) The devices have a bit error rate that’s tiny enough to expect no errors over the device’s lifetime. In addition, there are other sources of device errors where a file system’s redundant check could be invaluable. SSDs have a multitude of components, and in volume consumer products they rarely contain end-to-end ECC protection leaving the possibility of data being corrupted in transit. Further, their complex firmware can (does) contain bugs that can result in data loss."
(sorry for the edits, I finally found the paragraph my memory was referring to)