All I can say is it’s my opinion. TM is great when it works but I’ve had problems on and off for years. The one that happened most often was it would stop working when the drive became full. It would not delete old backups.
When this happened there was nothing to do except erase the drive and start again. Frequently the user would not notice the problem for weeks so the Mac went unprotected until they did. More recently, with TM on APFS drives, I have seen TM run repeatedly without actually recording any data. And I have seen it appear to run correctly for weeks but not be readable when I try to restore files.
Today I use Arqbackup to do hourly backups to B2 and a nightly backup to OneDrive. And I use Chronosync to do a local backup. And I don’t trust any of them.
So I verify my backups by restoring a couple files from each backup around once a month. That is my advice to everyone. Don’t trust your backup system. Test it regularly.
Once upon a time I worked for a software development company*, one no longer in business, where a programmer there had written a backup program. For all the company’s data. Had he bragged about how fast it ran.
It ran fast because it didn’t actually back up anything. He never actually tested it. And then there was a major hard drive failure. The excrement hit the cooling device that day my friends.
So yeah, test your backups. Regularly.
*Now this was a company where the VP of programming, at the annual user conference, pushed back against the users complaining about bugs, chastising them because they weren’t testing the updates sent to them. Testing was not a priority at this company.
Must have happened more than once. This was a PC based company.
Now, the three founders, including that VP, came from a company that used System36 and AS400 systems. And were accused of stealing intellectual property. They were acquitted, albeit wrongly in my opinion. But I knew none of this as it was happening. But it turns out I was involved.
During this, I was working for the company with the IBM systems, and was given the task of finding our code that functionally matched some PC code, from where I did not know at the time (I was the guy that coded in C on AS400). It wasn’t until I was long gone from both companies, and reminiscing about the good old days that this story came out.