Yes, sysupgrade is not atomic. But there is only a very small chance, to be caught with a bricked device, i.e. when having a power fail during writing new image to flash.
Having large fleets of openwrt devices, I am even doing fw upgrades over LTE. Can not remember of any problems.
Using custom firmware, based on "official" openwrt, for RUT955 and other openwrt devices.