I use both, no regrets. My DS918+ acts as a backup repository for my VMware VMs and bare metal PCs via Active Backup for Business. It's also a Time Machine repo for my Macs. I also use TrueNAS Scale on a dell T330 which acts as my media repository and shared VM storage.
The advantage of the Synology for me is its application ecosystem. Active Backup for Business is by far the cheapest way for me to back up all my VMs and bare metal Windows PCs (I have a lot, more than even the Veeam NFR license would cover.) Active Backup deduplicates my backups across the entire dataset, which is awesome. I also use the Hyper Backup to push some of my more important VM backups to Backblaze B2.
The advantage of my TrueNAS system is it cost significantly less than the Synology, was very easy to add 10Gbit fiber networking to, and has iDRAC. ZFS is also awesome. I also run a virtualized TrueNAS Scale instance, but its only purpose is to serve as a mirror for the primary TrueNAS system and the Synology.
Honestly, not everything needs to be a firsthand experience to know it’s not something I want to do. Hosting my own email is definitely something I’m good with living through others vicariously.