For two, if you like power adapters going into boxes out of which usb cables to go more external hard drives, a Pi may be fine. If you want one neat box to tuck somewhere and forget about it, they aren't.
But then people buy Intel "NUCs" where the power adapter is larger than the computer box...
And three, the latest Pis have started to require active cooling. Might as well go low power x86 then.
External drives were on sale, I bought several and setup with a RPI. Lots of headaches. It took effort to iron out all the USB and external disk issues. Had to work out alternative boot. Had power adapters fail for the RPI. Had to enhance cooling. etc. Kept running into popular Docker containers still not having aarch64 variants.
I finally replaced the RPI with a used Dell SFF. Kept the USB drives and it's been solid with similar power draw and just easier to deal with all around.
Though I am considering going back to a tower, shucking the drives (they're out of warranty) and going back to SATA.
With a proprietary on-disk format you can't exactly hook them up to any random controller and expect it to work: either you find a new one from the same controller family, or your data is gone.
You say that like it’s a mystery why people by then but NUCs are fantastic little PCs.
The power adapter is just hidden under the desk whereas the NUC is sat on the desk (or behind the monitor/TV).
It’s the same as with Mac Minis and Apple TV. And other devices of that ilk.
There seems to be a Windows-only update tool available that might fix it, but that's rather inconvenient when it's used as a server running Linux! No update available as a standalone boot disk or via LVFS. So I haven't gotten it fixed yet because doing so involves getting a second SSD, taking my server offline to install Windows on it, just to run a firmware update.
The Mac Mini sits on your desk and there’s nothing under it.
It's of course more work to set up than synology, and if you want a neat box, you have to figure that out yourself
However, in my experience with a Pi 4, the issue is encryption. The CPU simply isn't fast enough for 1Gbps of AES! Want to use HTTPS or SSH? You're capped at ~50Mbps by default, and can get it up to a few hundred Mbps by forcing the use of chacha20-poly1305. Want to add full-disk encryption to that? Forget it.
The Pi 5 is supposed to have hardware AES acceleration so it should be better, but I'm still finding forum posts of people seeing absolutely horrible performance. Probably fine to store the occasional holiday photo, but falls apart when you intend to regularly copy tens of gigabytes to/from it at once.
1293685061 bytes (1.3 GB, 1.2 GiB) copied, 5.14336 s, 252 MB/s
which is good enough for me on magnetic disks
It apparently hit 387MBps for a few hours while running the montly raid scrub. I run luks on top of mdraid though so the raid scrub doesn't have to decrypt anything.
scp to write to the encrypted disk seems to get me something in the 60 - 100MB/s range.
What you are describing is a hobby item for enthusiasts who want to enjoy tinkering, setting something crazy up and constantly debugging it. That is very much NOT the market of people who buy Synology products.
Synology is not a JBOD RAID. It is an appliance that does many things (storage, services, web access, etc.) and can automatically keep itself up to date with no additional contact necessary. It can be hooked up to a UPS for resilience. If you have a problem there are support forums and online articles. It can be synced to other Synology devices at other sites. Etc., etc.
A pile of RPi is none of that.