I too have a Synology NAS (DS1515+, raid 5) and can't recommend them strongly enough. It did have the boot issue due to some Intel Atom CE processor bug (I forget the details) where a boot config pin driver burns itself out over time - but Synology replaced it overnight. They sent me a preconfigured replacement chassis with the same firmware version as mine, Fedex overnight, and all I had to do was move the drives and memory expansion over, and power it up (forget if I had to do anything else). Then sent back the old unit in the same box, using the prepaid label they included. If this one lasts 4-5 years like the previous one then I'll probably be ready for a replacement around that time anyway.
I back up all important data to pair of external USB drives; one drive is plugged in and the other kept with a friend who lives a few blocks away. Every once in a while, like after I've added a bunch of photos and videos (of personal value) or something else important, I'll swap backup drives. The builtin backup software handles it perfectly. The bulk of stuff doesn't get backed up, especially things I can recreate as needed, like a significant assortment of toolchains and VM images for various uses. Other than that I'm not in the habit of torrenting or downloading the Internet, so don't really have all that much. I do keep installers and CD/DVD ISOs for more obscure or old versions of software. (Like for example the software for my Imacon scanner, along with a Mac Mini G4 to run it on and a FW400-to-SCSI bridge, drivers, and a complete image of the MMG4's drive.)
I do sync a bunch of stuff to it - git repos, google drive, and so on, but I don't back this up. I just keep it there to have a local copy, in case. Whenever I order any parts I save the datasheets and 3D models to google drive (Mouser these days provides step models). I really only back up what's impossible to redownload, recreate, reinstall, etc in worst case. This data set is surprisingly small...