Overlay on separate partition

Two questions:

  1. Why devices (octeon/ubnt_edgerouter, x86-64/generic pcengines apu) with mmcblk use ofsetted root overlay loop mount instead of a separate parition?

  2. Is there another way to use a separate partition for overlay other than pass FILES=dir to imagebuilder with a dir/etc/config/fstab file with a section like this?

config mount
	option label 'rootfs_data'
	option target '/overlay'
	option enabled '1'
	option enabled_fsck '1'

counter question, are you sure it applies to ext4 too ?