These links may help in understanding Flatpaks, where they are used, their safety, whether you can use them on Ubuntu (you can), the fact that snaps are closed source, and many other trivialities.
Ubuntu may come with Snap by default but you could still enjoy the Flatpak universal packages on it.
itsfoss.com
https://flathub.org/en-GB?ref=itsfoss.com ... (this is The Linux App Store)
https://itsfoss.com/flatpak-guide/ .... Using Flatpak on Ubuntu and Other Linux Distributions [Complete Guide]
Flatpak is a universal packaging format from Fedora. Enabling Flatpak will give you access to the easy installation of many Linux applications. Here's how to use Flatpak in Ubuntu and other Linux distributions.
As far as I can see, there are no downsides.