A rough summary is: Unless you work for Canonical, Snap doesn't really have much going for it over not just Flatpak, but normal package managers. But, unless you install a distro built around it (like Fedora Silverblue), nobody's forcing you to use Flatpak.

So yes, Snaps and Paks can waste similar amount of disk space and take similar amounts of time to start up, but the biggest difference is, Ubuntu has sort of woven Snaps deep into it, replacing debs for both popular applications and system components. A snap-ized Ubuntu boots slower and runs slower for basically no advantage to you as a user, compared to the same machine running something like Debian or Mint.

Now, obviously, sometimes it's worth the tradeoff. Maybe there's an app you don't trust very much, or maybe nobody has ported it to your distro of choice yet. Or maybe you run a particularly old distro, like Debian-Stable, so the version of the app in the repositories is a few years old and you want a new one. But then you can make the choice to install the Flatpak version -- unless a distro is deliberately built around it (like Silverblue), you aren't just going to upgrade and find a bunch of pieces of your system were replaced with Flatpak. But that's exactly what happened with snap -- we upgraded Ubuntu and suddenly there's a half-dozen snaps running at boot, and no way to disable it other than leave the distro.

...snaps provide even better isolation and sandboxing, and work in CLI apps.

Can you be more specific with this one? Because AFAICT, there are already CLI apps distributed as flatpaks -- it's not the best experience (you have to set up an alias), but it can be done. And as for isolation, both seem to do similar amounts of sandboxing, except Flatpak gave us portals (which hopefully Snaps are picking up too) to make it a little easier to implicitly grant access to things the user clearly wants -- e.g. if you pick something with an 'open' dialog, you probably want to give the app access to that file.

So that's for the average desktop user. But for a sysadmin, I think you're underselling the utility of a self-hosted repo. With Flatpak, or even with Debian, you can ship your own app on your own servers, particularly useful if it's (say) an internal-only thing that wouldn't make sense to just publish to all Snap users. You can have a giant caching proxy to limit bandwidth use, and you can also do periodic backups of that repo, or control which machine gets updates when. Basically, remember that time Crowdstrike bluescreened all those Windows machines? With Ubuntu, you're counting on Canonical not to do that, but you dont' actually have much control over it yourself the way you do with an open-source repo.