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.