I recommend you do a test restore every week or two for a while. My APFS TM backup was fine the first time I did a test restore. It was unreadable a couple of weeks later.
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After a little calm reflection, I have come to some conclusions about Time Machine. They may be wrong, but they may be useful starting points. I just wish there was some way of getting to talk to anyone in Apple about some of these things.
Conclusion 1: Time Machine is slower and less since the switch to SMB for talking to networked drives.
This is rather more of a hypothesis than a conclusion. However, it kind of fits in with the time that I started seeing problems, both with speed and with backups failures.
Conclusion 2: Time Machine problems have been amplified by the use of larger drives.
This is kind of obvious, really. My original MacBook had a 256MB SSD. My next two had 512MB SSDs, and my latest one has the full 1TB. More data to backup, so the backup takes longer to complete, so a bigger window for network problems. And no, I do not wish to return to tiny SSDs!
Conclusion 3: Time Machine is really only a problem on laptops.
I have two desktop Macs (a Mac Mini and an iMac) which rarely present problem with backups. Time Machine just works! But then, a) it is easy to keep an external drive connected for backup purposes, or if using a networked drive the machine is on a wired network connection, so there won’t be any network problems, will there? Similarly, if I connect an external drive to my MacBook, or use a wired connection (and disable wifi to ensure the wired connection gets use!) then backups complete more quickly (as I’d expect) and more reliably. But who wants to do either of those with a portable computer?
Conclusion 4: Time Machine as currently implemented is not fit for purpose for use on laptops without radical change.
Why? Because one doesn’t want to connect external drives or wired network connections to a laptop. Besides, a laptop is not left to run all the time - one uses it, then closes the lid to preserve the battery. Or if you don’t close the lid it will go to sleep anyway. When the laptop sleeps (or, more accurately, gets to a certain level of sleep), the wifi hardware is powered down, and the connection is lost. It is my hypothesis that a) with smaller SSDs taking less time to backup, this used to be less of a problem because for any given pattern of use there is a smaller chance of interrupting a backup than with a larger SSD, taking longer to backup. Further, I hypothesise that SMB is more susceptible to network interruptions than was AFP.
On the assumption that I am correct, it follows that Time Machine is not fit for purpose on a MacBook because instead of completing multiple backups per day it is frequently not possibly to complete a backup for multiple days (or weeks!) at a time without making special provision for the machine to be left online and on power whilst the backup completes - which negates the whole point of Time Machine - that it should just work reliably in the background without the user being particularly aware of it.
In order to fix this, Apple need to come up with some way of achieving a session-level connection between the MacBook and the networked disk that is tolerant of (or capable of dealing with) interruptions to the connection.
If there’s anyone in here from Apple, I’d love to talk to you about it! 
Colin
Okay, those are hypotheses 
TimeMachine is the Windows ME of Mac software. Much like Apple Mail, I can only assume that none of the developers actually use it.
For me, it fails every six-nine months, requiring starting over. This is true whether on my NAS, or on directly attached drives.
I stopped using it a couple of years ago.
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This is not mere speculation, and it is the root of the problem, as researched and documented by Howard Oakley.
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Thank you for that link - it’s nice to find one’s speculation confirmed by so eminent a source!
Meanwhile, my solution on my MacBook (and it’s only a partial solution, but it certainly helps) is to keep as many of my files as possible on my Synology NAS using the Synology Cloud Station Drive software. This is, in effect, a personal dropbox service, but with the data stored on my NAS rather than in the cloud. This set of data is also shared (synched!) on one of my desktop Macs, which saves it via Time Machine, whilst the MacBook excludes it from Time Machine. This means that the Time Machine load on the MacBook is much reduced. It’s far from perfect, and I’m aware of its shortcomings, but it makes things a little more usable! Fortunately my data isn’t particularly fast-moving …