Actually not.
While the same coating was used for the longest period of DD floppy production, there's a problem with the way of writing. Once a disk has been written to just a single time with a HD drive (e.g. factory formatting) the 'harder' magnetic imprint cannot be completely wiped out by a 'softer' DD head. (This is valid for DD disks used in HD drives, too!) Over time, the HD bits can creep back, wiping out the newer but softer DD bits.
So, for longevity - as most Amiga disks tend to be used - it's not advisable to use HD disks. One way to get around this is to use a HD drive even for DD media - but don't mix them with multiple drives. Probably the best way to store important data is on harddisk and backup to quality CD/DVD.
