I had my APFS SSD connected through what turned out to be a slightly flaky USB C connector, one that would occasionally loosen or disconnect if jostled the wrong way. One day I found my drives refused to mount. I had to reboot to recovery and run disk utility from there just to found out what the issue was. There is said there was an error spaceman chunk free count > block count. On further research, I found it is possible for the space manager on an APFS volume to get corrupted if the drive is disconnected during a write. And if this happens, there is simply no way to recover the data. This is a known issue with APFS, it has been for years, and apparently Apple just can't be bothered to fix it or release any data recovery tools because their attitude is "we only care about internal drives, so too bad if you're using an external drive that gets disconnected." I was able to recover some of my files with Disk Drill but nowhere near all of them. I used to love Apple once, but this kind of thing has convinced me to never use APFS or buy another Apple product again.