Not a fan for a few reasons. Flathub (as far as I know) works on the app store model where developers offer their own builds to users, which is probably appealing to people coming from the Windows world who view distros as unnecessary middlemen, but in the GNU/Linux world the distro serves an important role as a sort of union of users; they make sure the software works in the distro environment, resolve breakages, and remove any anti-features placed in there by the upstream developers.
The sandboxing is annoying too, but understandable.
Despite this I will resort to a flatpak if I’m too lazy to figure out how to package something myself.
This
I use Guix as my “default” distro because I value software-freedom and reproducibility. It fits my needs very well, and I make sure to buy hardware that works with it instead of expecting it to work with whatever I throw at it. For my Windows gaming machine I use PopOS as the replacement OS instead of trying to beat Guix into serving that purpose, because PopOS is better suited for that role, and I have different expectations for it.
It’s okay if something doesn’t meet your needs, that doesn’t make it bad, just means it’s not the right thing for you. There’s like hundreds of distros for Windows gamers, let us free software zealots have ours too please.