The major downside to ZFS-based systems like TrueNAS is that for a home or small business user you can't expand the storage with a new drive when you're running low on space. It's designed for data centers where you can afford to build a whole new array when you need more storage.
With Synology you go "oh, I'm down to 1 TB free, well there's this deal on a 10 TB drive, pop it in, now I have 11 TB free"
With Synology you go "oh, I'm down to 1 TB free, well there's this deal on a 10 TB drive, pop it in, now I have 11 TB free"