Since power-loss is (hopefully) equivalent to an atomic storage snapshot, a database which doesn't produce a consistent backup via snapshots can't be safe against power loss and thus lacks the durability property of ACID.
What's unsafe is using a naive file copy tool (e.g. `cp`), which non atomically copies a running database.
Without taking the aforementioned steps I specified, power loss is not an atomic event, because most databases nowadays rely on the fsync(2) system call to tell them when the I/O has completed. If the fsync(2) call is unreliable because it is lying about completing the requested I/O, the RDBMS stands no chance of guaranteeing atomicity. I am sorry.
What's unsafe is using a naive file copy tool (e.g. `cp`), which non atomically copies a running database.