Earlier quoted context omitted.
For one rPIs are severely i/o limited still. May be fine with one ssd. 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 ac…
If you use a couple of magnetic disks, the pi is fast enough. The disks will be the bottleneck. There are sata cards that allow up to four magnetic disks, and where you power that card which in turn powers the pi. It's very doable. 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.