So today if a NiFi node dies the data should still be recoverable unless the disks/storage itself is dead. So losing a node should not cause data loss but it does impact availability.
Currently the way to achieve rapid availability in the presence of node failure would be through having the key repositories (content, flowfile, provenance) all mounted in such a way that you could remount them to another host. This is a typical infrastructure/OS level approach that can be managed outside of NiFi itself. It is admittedly non-trivial to get right.