I'm sure official Flatpaks do better on average, but that narrows the selection a lot. And most third-party packagers raise my suspicions in a way that, say, the Debian packaging team does not. You should see my eyebrows go up when I cruise dockerhub!

I don't know what the right answer here is, but for me it's not Flatpaks or Snaps. Trusting the packager (and the org behind the package system) is a huge part of it. I breathe a sigh of relief every time I see the software I need is in Debian's repos, even if that means they may not be optimally sandboxed.