Like many here I had the exact same issue mentioned by the poster, Time Machine on Synology just kept failing, with incredibly unhelpful error messages that basically said, let's trash and backup again.
I used AFP which was recommended back then and that worked really well for years (since 2011). But since (maybe) Catalina, issues started creeping up and it would just randomly fail. It used to be once in a while, then it became a weekly occurence before I gave up.
Samba isn't better, my mounted shares get randomly disconnected overnight fairly often too (even now on Monterey), and switching from the old Synology to a fresh dedicated NAS machine didn't change a thing.
At that point I think in general it's just "local networking" that became less reliable around that time, whether it's some power saving feature, or something else up the stack, I don't know.
The only scenario where Time Machine works flawlessly for me is using an external SSD drive for backup, formatted as APFS. At least for now.
At this stage I think Time Machine is barely fit for purpose for backing up over the network. I've lost days on this issue over the years too.
I have always been totally confused as to whether I should be using AFP or SMB (tried both). As others have said, SMB often seems very unreliable, and AFP is supposedly being deprecated...
Mounting network shares from my Synology to Mac(s) is never flawless either. As other comments have noted, this experience is very much worse than what it used to be like in Windows (not that I've mounted network shares in Windows for a while).
And same exact experience on mounting shares by the way.
When setting up my new NAS last week, I ended up booting a Windows PC to check if Samba was correctly configured because macOS kept throwing weird inscrutable errors semi randomly.
I know it's a terrible protocol but it certainly became worse on macOS on the past 3 years-ish.
I have a couple of 6tb usb disks formatted as a Btrfs RAID 1 volume plugged into a Rpi4, it's not the fastest but it has been reliable for over a year. I always make sure I stop the backup before pulling the cord on my laptop though. My wife hadn't been doing this until recently and she has had more problems with both over the network and directly connected usb disks; she has both. At some point I'm going to create a duplicate setup in my house and send Btrfs snapshots over the internet to have an offsite backup, but I haven't got round to this yet. Currently our offsite backup is just taking a laptop home...
I used AFP which was recommended back then and that worked really well for years (since 2011). But since (maybe) Catalina, issues started creeping up and it would just randomly fail. It used to be once in a while, then it became a weekly occurence before I gave up.
Samba isn't better, my mounted shares get randomly disconnected overnight fairly often too (even now on Monterey), and switching from the old Synology to a fresh dedicated NAS machine didn't change a thing.
At that point I think in general it's just "local networking" that became less reliable around that time, whether it's some power saving feature, or something else up the stack, I don't know.
The only scenario where Time Machine works flawlessly for me is using an external SSD drive for backup, formatted as APFS. At least for now.