You don't (typically) lose the data on the ephemeral drive across a reboot but you definitely can (and do!) when there are more permanent hardware failures. (They really happen!) That's why PlanetScale always maintains at least three copies of the data. We guarantee durability via replication, not by trusting the (slow, network-attached) block device.
I did an interview all about PlanetScale Metal a couple of months ago: <https://www.youtube.com/watch?v=3r9PsVwGkg4>