Flatpak sandboxes too, but it at least is fully Open source, you can create your own Flatpak Repository and add it to your flatpak to grab and install new packages you made yourself. For Snap… it implements not that good into the desktop I heard but may have changed, you can’t create your own repository nor see the servers code as there is only 1 single server for Snap, and its canonicals Closed Source Snap Server. But hey, it at least got the super cool Hologram Open Source Sticker on it, because the client is at least Open Source… No thanks
I use PrismLauncher (Minecraft Launcher) inside Flatpak for example because it sandboxes the app so no stupid mod can infect me that easy now, haha! But generally its kinda comfy to use Flatpak because it has less dependency issues compared to Native Packages because Flatpak has its own Packages which Flatpak Apps can share to each other. Snap on the other hand can’t have dependencies shared between Snap Apps so they all have duplicate dependencies.