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.