Interesting, could you please elaborate?

1. What exactly is this "built in sandbox", and what does it protect against? How does it compare with Flatpak disallowing access to filesystem?
2. Could we get a source for the claim of sandbox being crippled? Or more details? Documentation? Build scripts?

I had a look at flatpaks I have installed:

• Firefox (org.mozilla.firefox): no access to ~

• Thunderbird (org.mozilla.Thunderbird): no access to ~

• Element (im.riot.Riot): no access to ~

• Beyond All Reason (info.beyondallreason.bar) - no access to ~

• Steam (com.valvesoftware.Steam) - no access to ~, and (best of all) Steam runs a ton of untrusted code in games, which will inherit this restriction.

• Wolfenstein: Blade of Agony (com.realm667.Wolfenstein_Blade_of_Agony) - no access to ~

• Chromium (com.github.Eloston.UngoogledChromium): allows access to ~ by default. It's one click to disable, or I could shop around for another one, like org.chromium.Chromium.

• OpenTTD (org.openttd.OpenTTD) - allows access to ~

Thus, yeah, some apps neglect to restrrict ~, thankfully it's easy to fix. It's not a disadvantage, though, it's a lack of advantage.