There is very low chance this is "something wrong with APFS on macOS". But there is something wrong for example with "Time Machine" backups reliability and breaking changes. Not supporting perfectly good hardware just to force users to buy new ones is also not nice. For example you can't easily move TM backup between disks or between hfs+ and apfs because time machine on hfs+ use hard links and on apfs snapshots. One time I've lost whole TM backup, not because of drive or filesystem failure but because of TM being unable to manage edge cases which arrise during network backup.
Is it that bad to buy a new backup disk when you change file systems? It's been many years ago since HFS+ got retired.
You're right, in the grand scheme of things, buying a new disk every few years isn't a major hardship but obstacle. My point wasn't really about the cost of hardware or not being able to solve problem, but rather the brittleness of the software, documentation and usabillity.
The difficult backup migration is just one symptom of a larger problem: Time Machine isn't robust. It has sharp edges and can fail in ways that are hard to recover from, like the network backup I lost. That lack of reliability and breaking changes not just in backup software but in whole ecosystem is the real issue for me. Apple "obsoletes" things in its own timeframe regardless of userbase. I've got enormous respect for Apple products and people who are building them and with them. I wish Apple would have "LTS" version of MacOS and better documentation - often tools exist but aren't easy accessible.
> There is very low chance this is "something wrong with APFS on macOS".
Why do you consider the chance of that to be low? APFS is functional and I trust to not eat my data, but it isn't a battle tested high performance file system that's done the rounds in the arena like, say, XFS.