Jon ran a poll on Mastodon with regards to what linux packages people mainly use.
722 votes =
58% DEB
43% Flatpak
24% RPM
7% Snap
Jon also mentions Flatpak specifically:
At @Vivaldi we clearly like to support the packages our users want. At the same time adding packages is not just a question of making the packages, but also a question of testing, which can be quite time consuming.
The issue with Flatpak is that the Flatpak sandbox requires us to disable the Chromium sandbox, which is not optimal.
If there is a way around that, that would be great!
Disabling the Chromium sandbox is a potential security problem and requires changes in Chromium.
Source