QNAP had a minor version upgrade a while back that re-enabled auto-updates that had explicitly been disabled and there was a major version update (v4 to v5) not too long after that. I was intentionally holding off on upgrading to v5 because there was a Samba related issue that broke my primary workload. I almost got burned by it. Yesterday I got an email from one of my QNAPs saying an automatic firmware update was scheduled for 00:00 today even though I have auto-updates turned off.

I don't think it actually updated today, but I don't think they have their act together when it comes to managing updates and I'm not willing to depend on any of their stuff.

Synology is more complicated and ultimately comes down to the use of BTRFS. I don't know a ton about filesystems, but, the way I understand it, BTRFS allocates extents and then puts blocks into those extents. Depending on your workload, you can end up with orphaned blocks in those extents that prevent space reclamation (because it reclaims extents, not blocks) and it can result in runaway space usage. Search for "BTRFS missing space".

I may not have gotten that 100% correct, but I think the basic idea is close.

My workload (backup storage) overwrites random blocks in existing files and that's one of the scenarios that exacerbates the issue. I've ended up with empty LUNs on a Synology that are "using" TBs of space on the containing volume.

The Synology can also have a pretty complicated "stack" by the time you get your data onto it. I think I had an image based LUN on a BTRFS volume on mdadm RAID1. That was achieved through the GUI without making any crazy choices AFAIK.

I went all-in on Synology a few years ago. I can manage a bunch of drives myself, but that's not how I want to spend my free time. The Synology just sits in a corner of the house, doing its thing 24/7/365 without me futzing around with it.