Gregory's ServerSnap predates Flatpak by about a year, as I recall, so it was a novel solution to a problem when it debuted. (I mention this because Canonical frequently gets accused of NMiH over software that predates similar projects.)
Flatpak also doesn't work great with server or command line tools; it's mostly geared towards GUI desktop applications.
Flatpak didn't offer isolation like Snaps did until very recently.
⭐ Snaps + Ubuntu Core for network/computer/VM appliances.