I've never heard of anyone getting an unsafe package from flathub, but they certainly aren't all as thoroughly vetted as stuff from a well maintained distro. Any major package is almost certainly fine, but if you're downloading something obscure I'd use Flatseal to make sure it's very well sandboxed, just in case.

They've also recently added verified checkmarks to the website for flatpaks that are officially maintained by the developers of the app, so that's another thing to look out for.

I don’t remember anything about flathub, but the Ubuntu snap store had some malware a while ago

https://www.linuxuprising.com/2018/05/malware-found-in-ubuntu-snap-store.html?m=1

Apple, Google, Canonical all had malware in the stores.

Nothing can ever be always secure.

use flatseal to restrict access helps if worried

They aren't inherently safe. I don't have any examples of Flatpak packages off FlatHub being poisoned, but FlatHub does allow "community" maintained packages - as in, someone unaffiliated with the development team of an app packages and publishes the app to FlatHub. That would seem to be a really good place to get into a supply chain if you were a bar actor.

Even disregarding the trust issues with Flatpak packages made by random people: Packages often contain versions of some libraries in order to not depend on the distro's. If there are security vulnerabilities in a library then the distro maintainers usually fix it very quickly (if not go find a better distro) and it's fixed for all packages on your system that depend on it. But this doesn't apply to Flatpak where the package providers have to update the libraries in their own package - and the track record isn't great. Sandboxing doesn't help if that vulnerability leads to wiping your home directory.

Flathub apps will likely be removed quickly if they're found to be malicious. They're slightly more unsafe than official repos, but you should be fine. Make sure to carefully check apps with like 4 downloads though.

No, they are not always safe.

Be picky about what you install, and vigilant about permissions.

Flathub is likely safer than most other places to get flatpaks from, certainly safer than just some random repo you find on some guy's website somewhere, but no software source is guaranteed to be 100% safe.

At https://blog.frehi.be/2023/04/23/the-security-risks-of-flathub/ someone has published an article about Flathub in which he addresses a few problems.

Therefore, the answer is that Flathub is not always safe to use. However, I do not know of any package source that is always safe to use. Is Flathub more insecure than other package sources? I can't answer that because I don't use solutions like Flatpak, AppImage etc. myself.

Not 100%, it's not very hard to push packages to Flathub.

The general community is probably going to catch any issues that pop up extremely quickly. Like my main machines are all on whitelist firewalls residing on external devices. If any software tries to make odd connections, the connections will get dropped and logged. I wouldn't hesitate to report anything odd. I don't run sketchy proprietary junk for the most part.

On the contrary, downloading files from flathub are never safe because it does not verify signatures, unlike secure package manager like apt

I think so. In some cases the flatpaks are prepared by the developers themselves. This isn't in itself a sign of trustworthiness, but if a dev were to sneak malicious code in somewhere and it were found out... Well, the internet is the courtroom, and the public the jury, right?

But, it is a piece of software, and you never know what one little dependency can do. Same can be said about repos.