No. It will boot the previous kernel, but the user experience will be at least suboptimal if some packages have already been removed during the upgrade, but the upgrade stopped at some point because a downloaded package was corrupt, leaving lots of dependent packages unconfigured. In case networking doesn’t work, it’s also inconvenient to manually download the affected package on another machine and transfer it with a usb stick onto the computer to restart the upgrade.
I don’t know about arch but my system usually boots fine after an upgrade. (Gentoo here)
If the update is successful. If there are failures in critical steps, well…
Doesn’t it roll back to a previous state then?
No. It will boot the previous kernel, but the user experience will be at least suboptimal if some packages have already been removed during the upgrade, but the upgrade stopped at some point because a downloaded package was corrupt, leaving lots of dependent packages unconfigured. In case networking doesn’t work, it’s also inconvenient to manually download the affected package on another machine and transfer it with a usb stick onto the computer to restart the upgrade.