It’s not quite that simple.

Each package can choose one from a handful of runtimes to use, each of which include common dependencies (like gnome or qt libraries), and if multiple flatpaks use the same runtime, that runtime is only downloaded once.

It is less space efficient than your typical package manager, but brings other benefits like sandboxing.

Most dependencies are bundled in the “runtime” images, and it uses file deduplication to reduce the size of the dependencies, but it’s still a little more than a normal package manager.

Not necessarily. GNOME and KDE dependencies and “base system” for flatpaks to run in are flatpaks themselves so apps that depend on them will not use duplicated dependencies. Storage usage may not be as efficient as using a traditional package manager but you don’t install a new OS per app either.

on a desktop it might not be significant but I tried using flatpak apps on a device with very limited root emmc storage (16 GB) and ran out of space really fast. Its really common to see a couple multi-hundred-megabyte library downloads for each new app IME.

I like them for some stuff but there are glaring issues that I don’t like. I’ve posted about it before, poor integration of apps/not getting the right permissions is a big problem, the people packaging them don’t often do as good of a job as someone like a distro maintainer.

But admittedly my experience using it probably isn’t representative (pop os through their shop and arch on a mobile device). Neither were amazing, but not having to compile shit myself or install with an untrusted shell script was nice for some apps. Without some significant improvements it’s not a good replacement for a distro’s package repos but it might be a good way to broaden the available applications without having to maintain 10x more packages.

Flatpak as a dependency system that allows use of specially packaged library type flatpaks. This significantly reduces the needed disk space.

Essentially yes, if you start using lots if older applications or mixing applications that use many different dependency versions, you will start to use lots of extra disk space because the different apps have to use their own separate dependency trees and so forth.

This doesn’t mean it will be like 2x-3x the size as traditional packages, but from what I’ve seen, it could definitely be 10-20% larger on disk. Not a huge deal for most people, but if you have limited disk space for one reason or another, it could be a problem.