Needing to unmount was a property shared with other operating systems, but early Unixlikes used to have silly problems. For SCO Xenix I was advised (by a SCO employee) to shut down using the following formula:
sync
sync
haltsys
The second sync didn't do anything the first sync didn't, it was because on that platform sync returned immediately instead of blocking until the unwritten blocks had all been written, and it was there to slow you down. You didn't want to halt too soon...
I was used to doing something before shutting down on DOS though, because my first PCs had ST-506 interface disks and those usually didn't park themselves. You had to send them a command to ask them to do it, which most people did with PARK.COM. ATA interface disks would generally self-park. Some earlier SCSI disks would not, but of course eventually they all did. Almost no MFM/RLL disks would park themselves.