Is APFS fully supported yet?

Nobody covers APFS better than Howard Oakley:

So, as of High Sierra 10.13.3, APFS is the standard file system for SSDs which are only used by High Sierra systems, “can” be used on hard disks which are only used by High Sierra systems, but remains unsupported on Fusion Drives.

There are four major limitations to the use of APFS.

Essential reading.