resinOS is designed for embedded devices, already supports many of them running several architectures (all the way down to ARMv5). While I have heard that rancherOS can run somewhat on a raspberry pi, that's a long way from being able to properly function in an embedded environment (read-only rootFS, reliable networking and DNS stack, support for atomic host updates, etc). There's nothing we'd like more than to have someone make the OS for us, but at this point we're pretty sure we'll have to do it ourselves. The variance in the embedded world is simply too large for a cloud OS to handle it without major rearchitecting.
Transactional updates are pretty much standard for embedded devices that care about updates. Android Brillo, ChromeOS, Android, Snappy, and many many others employ similar strategies.
ResinOS does indeed have a smaller footprint, a broader set of supported architectures (Snappy only supports ARMv7 and above), and uses Docker instead of LXD+snaps.