Read only root partition? - Resilience to unexpected power loss?

Looking at embedded Linux and materials for building firmware for IoT, everyone speaks about have 2 read-only root file systems, for example:

By having 2 root filesystems and 2 kernels, you always have a working copy that is switched to only after a successful write. From what I understand this is somewhat similar to Ubuntu Core, except Ubuntu Core has only 1 filesystem, mounted read/write.

How does Ubuntu Core handle sudden unexpected power loss? - Isn’t there an increased risk of FS corruption that can be avoided by having a second root partition? - or am I missing something?