HFS+ vs APFS: What are the pros/cons of converting HDDs to APFS?
APFS (Apple File System) compared to HFS+
Almost all differences apply to Solid State Drives (SSD) and Hard Disk Drives (HDD) as well, while users of HDDs may not see an improvement in speed.
APFS disadvantages/drawbacks
- Incompatible with macOS 10.12 Sierra and earlier (including OS X)
- Windows drivers/tools to read/write APFS disks are not available (as of today)
- Currently not supported by Fusion Drives/HDDs (coming soon)
- Boot Camp doesn't read from or write to APFS-formatted volumes
- No AFP support: Volumes formatted as APFS can't offer share points over the network using AFP.
- Any Time Machine share points must be shared over SMB instead of AFP.
- Directory Hard links are not supported
- File names cannot contain unassigned codepoints in the Unicode 9.0 standard
APFS advantages
- Faster disk speed in general on SSDs
- Almost instant file and directory duplication
- Increased maximum number of files
- Space sharing
- Better crash protection
- Better date resolution: nanosecond timestamps
- Support for dates beyond February 6, 2040
- Sparse file support
- Larger max. number of files (2^63 vs 2^32)
- Supports Copy-on-Write (replaces journaling)
See also Apple's APFS Developer Guide and the Wikipedia's Apple_File_System.