my issue with snaps is honestly just that they are controlled too much by just one entity (canonical) and there is no reason for them to exist because flatpak already does everything they do.
My issue with snaps is also the power that Canonical has to fuck you over one day, because of the centralization that you mentioned, but also that their shitty fucking packaging format sucks ass and breaks everything but the most basic of apps. I've wasted hours trying to help people with their broken applications that were hijacked when they typed apt install whatever and "whatever" was actually a fucking broken snap package.
Flatpaks and AppImages actually do the fucking things they're supposed to. Snaps don't, and Canonical is pulling a Microsoft by hijacking your package manager.
Also, Snap sandboxing only works with AppArmor, so if you were hoping that all the breakage was worthwhile because you get sandboxing, you don't if you're on anything but a handful of distros 🙂