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Updated
This brand has a long-standing of reliability and trustworthiness in this category. They have excellent long-term support. Feature updates (e.g. the newest DSM WebOS) are often even pushed out for 10-year-old hardware - you'll find it hard to find that level of support elsewhere. This NAS has 2-bays, storage up to 2x108 TB (theoretically speaking), companion apps, and its UI runs in a WebOS-System within the browser (and is therefore not bound to any operating system).
SpecsUpdate
Pros
Pro Single-button press backup functionality (ports and button on the front)
This function is called USBCopy. You just need to attach your media to the front, press the Copy button (labeled C on this version) and wait untill it's done (no need to have it attached to any PC and no need to login to the NAS as well). By default, the backups get compressed to a single archive per backup and are named after their device name and timestamp attached (changeable within the settings). Older/other versions also had an SD-Card reader, this one doesn't (but it seems to have an eSATA port) so you might search for an alternative if you need the SD-Card reader.
Pro Company-level advanced BTRFS file system with built-in data integrity check
See further details on Synology's page
Pro DiskStation Manager OS is rock solid
DiskStation Manager OS is by far the best commercial NAS-OS out there when it comes to expandability, usability, and stability. No other system comes close, unless you build one yourself and use an Open-Source NAS-OS that you fine-tune to your needs.
Cons
Con Requires a bit of work if you want to get the most out of it
It's clearly not made for people that want to buy a device and just use it but those that take some time to set it up. Once you're done, it runs, and runs, and runs (till you want to do more, which you can since it has Package-Management built in that enables you to download new packages and extend the functionality of your simple NAS).