Flatpak apps usually come with quite open privileges, however the user can completely configure this themselves and restrict the access of an application to quite a reasonable degree. Unless you distrust the sandbox of Flatpak, I don't see a need for containers.
Worth noting that Flatpak's sandboxing is using the same container functionality of the Linux kernel as all the various other container tools. If containers are secure enough than so is Flatpak, assuming you've tweaked the applications sandbox settings to your liking.
