Flatpack is kind-of like PC-BSD PBI. It's halfway between traditional packages and containers. The next evolution of this concept is ostree. Check out Fedora Silverblue which implements rpm-ostree. Think of it as git for packages. Packages are bundled like git commits, an all-or-nothing approach.
Red Hat has been pushing their immutable O/S, an ostree based O/S, for a while now. Their CoreOS, the platform that OpenShift runs on, is an immutable O/S. (Yes, CoreOS is officially dead but it's still the OpenShift platform.) OpenShift is an on-prem cloud platform.
There is no such concept as ostree for FreeBSD or any other OS other than select Linux distros. It's a great idea for an O/S vendor where the vendor has control over package selection and management, reducing calls to the help desk (costing the vendor $$$).
This of course opens up the discussion of traditional packages v.s. packages that contain all dependencies (flatpak and at the O/S level ostree). There are advantages and disadvantages on either side. Personally, I've spent 50 years working with traditional packages (IBM mainframe, various UNIX, Linux and FreeBSD). I've worked with ostree lately. Ostree is more predictable for the uninitiated but what it gains in apparent end-user simplicity it loses flexibility, useful to a seasoned sysadmin. Though, more secure as there's less "wiggle room" to customize.