I’ve seen and experienced a variety of issues over the years, but my distrust of time machine is based on silent failures. I’ve been able to benefit from time machine backups when they were a convenient solution, but when they were really needed, I’ve seen them fail too many times. At one point, I discovered that my time machine backup had been silently failing to backup both photos and music for over a year.

A couple of years later, I helped a colleague piece his data back together after failures in his redundant time machine backups (kept at home and office) that he discovered after wiping/rebuilding his primary computer. One copy was missing the last months of data, despite him having connected the drive and watched it complete the backup just before wiping. The other was missing important folders from the home directory though it was fortunately up-to-date in all other places.

Time Machine is the only backup software I've ever used where it seems to routinely tell me that my entire backup is corrupted and I need to start again. This has happened on multiple Macs, multiple macOS versions and multiple backup storage devices, for many many years.

It seems that fundamentally, backing up over the network is not reliable. If I'm forced to connect a local drive to my laptop to backup (which is not very convenient) then I'd much rather use something like Carbon Copy Cloner where I can feel a lot more confident that I've got an actual usable backup at the end of the process. Or use something optimised for network backup like BackBlaze (or, my preference, Arq, which admittedly still has its own problems).

I guess what I mean by obscure though is there is very little in the way of UI for Time Machine. There are no easily accessible logs. No simple list of backup versions. Almost no configuration options. I get that it's very much a "black box" backup solution designed to be simple to use for everyone, but the lack of transparency about what it's doing, for me at least, just adds stress as I'm never quite convinced it's working correctly. This would be more forgivable if it was rock solid and reliable but my experience is that it has never been those things.

My most recent issue was with TM's use of APFS snapshots and somewhere between TM and macOS, vast portions of my SSD storage being "tied up" and not released properly back to the OS as free space. This caused all sorts of issues with eg. trying to run VMs and VMWare complaining that there is no space left on the disk.

The overwhelming feeling I get when using iCloud backups and Time Machine is a vague feeling of "uncertainty". Time Machine (for me) is flaky and iCloud backups are not backups in the truest form, in my mind anyway.

My favorite trick to watch is during a Time Machine backup it stops because the (attached) backup drive ran out of space. I thought it was supposed to clear out space as needed. Perhaps not.

It does but if the first attempt fails to clear enough space it’ll stop. You can kind of fix it by excluding some large folders or manually deleting backups.