The block thing is true but not really why. Flash is erased in 4 KB sectors and written in pages, so you can’t patch bytes in place. Delta updates are still possible, the device rebuilds the new image from old image plus patch into a spare partition. It needs extra code on the device and a patch tool on the vendor’s side. Most small vendors don’t bother.
The safety bit you want to ask about is whether they have two OTA partitions. Then the new image goes into the inactive one and the old one is still there if it fails to boot.