- For platter-based drives, APFS can cause serious fragmentation. I've found "drive thrashing" to be a problem with those platter-based drives I'm using (as boot drives) for Catalina and Big Sur.
- HFS+ (on a data drive) may be more "fixable" if the drive is having problems, because existing 3rd party diagnostic/repair software doesn't work well (or at all) with APFS.
- HFS+ can be mounted, viewed and accessed on older Macs (if need be). APFS can't.
Perhaps with SSDs, APFS will do fine with data drives.
On my own 2018 Mini (which runs Mojave), I have the internal SSD partitioned 4 ways:
- Boot - APFS
- Main - HFS+
- Media - HFS+
- Music - HFS+
But I guess that's "just me"...
Never have had any issues with APFS on my external platter drives or on my external SSDs. I have no old machines hanging around the house now, the two which are here and which are used regularly are both APFS anyway so no need to be concerned about trying to use an APFS older drive with one of them anyway. Actually, as I'm typing, my 2018 machine is happily hosting two older platter drives (both with APFS) -- one is copying data from the other. I don't use any external drives as boot drives, so obviously I wouldn't have any issues with regard to that anyway. Whatever works for each individual is what counts, right?
If your Mac is running on a version of the OS which uses APFS, then format the external(s) to APFS. If it is running on an older version, then use HFS+.
For platter-based drives, APFS can cause serious fragmentation. I've found "drive thrashing" to be a problem with those platter-based drives I'm using (as boot drives) for Catalina and Big Sur.
There have been a lot of posts about when to use APFS. It was designed for SSDs which have almost 0 latency when accessing random areas of the SSD. Hard disks have to move the physical heads to the appropriate track which is significantly slower.
APFS was designed with the low latency of SSD's in mind. It uses a lot of pointers.
"While APFS is optimized for the Flash/SSD storage used in recent Mac computers, it can also be used with older systems with traditional hard disk drives (HDD) and external, direct-attached storage. macOS 10.13 or later supports APFS for both bootable and data volumes."