Yes that seems unlikely. Almost all Unix software uses buffered IO, including several other databases or database-like things including package managers, DNS s

Yes that seems unlikely. Almost all Unix software uses buffered IO, including several other databases or database-like things including package managers, DNS servers, mail servers, ..., and MySQL does too by default. It even uses it when you choose O_DIRECT (though I'm not sure if that's exposed to write-back error eating risk or not). That said, as several people have said, this is a very unlikely failure mode. When filesystems die, you know about it. The type of transient failure required to lose data this way is quite unlikely.