After converting my external SSD drive to APFS I found that it was having errors and wished to reformat it and restore. Solution: use Mac OS Extended instead of APFS on external hard drives, until Apple fixes this limitation. This makes backups of such drives problematic.
I have an internal SSD that upgraded to 10.13 and continued to the current 10.13.6. Many others claim that when you install 10.13 on an SSD, you will be converted to APFS whether you want it or not.
Many web pages claim that when you install 10.13 on an SSD, you have an option to convert to APFS.
