HFS+ vs APFS: What are the pros/cons of converting HDDs to APFS?

Question

I've got a few USB 3.0 HDDs I use for backup and game storage, all formatted in HFS+.

What are the advantages/disadvantages of migrating to APFS on an HDD?

Answer 1

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.