Trivially on their (and qnap's) amd64 systems at least. There are some quirks where they are more similar to an embedded system than a PC, but it's not a big deal. Things like console over UART (unless you add a UART) and fan control not working out of the box, so you set it to full speed in bios or mess with config.

Debian has docs on installing on at least one model of their arm boxes: https://wiki.debian.org/InstallingDebianOn/Synology

I run Debian on a few different models of qnap because their hardware occupies a niche of compact enclosure, low noise, and many drives.