Regarding immutable distros
It is my impression an immutable distribution is more about package transaction management and native rollback ability like in case of corrupt packages or system than about security. Would the security benefits alone justify using Silverblue as a desktop OS? - Fedora Discussion is an interesting discussion to read. Also Linux kernel itself has many vulnerabilities. A kernel exploit can just remap the filesystem to be mutable again, and do changes without revealing transactions, immutable distros can’t help here.
Mandatory access control, especially SELinux, is the really kicker for security, if properly and extensively configured. Android is a good example. Fedora has it enabled, but only provides a basic ruleset, so not sure, if that’s a game changer.
Regarding secureblue
Developers seem to be laser-focused on security, and sometimes neglecting privacy. Last times I checked, Trivalent tried to initiate lots of connections to Google, similar to Chromium or even Chrome. Ungoogle-chromium in contrast does not initiate any connections (but is not included in secureblue due to weakened security/disabled security features). Some connections like certificate verifications certainly are useful, but I expect full control on a privacy-focused system with rather opt-in to any of Google connections.
Another big Cons is that Chrome-based browsers use Manifest V3 for extensions, which makes ad-blockers like uBlock Origin much weaker against tracking, ads and blocking domains. If I read correctly, Firefox and derivates (Librewolf, Tor Browser) cannot be installed natively at all (there is a Firefox Flatpak, but common argument is a weaker sandbox).
Last there are software restrictions in place. Certain VPN software can be installed, but rather as customized installation workaround, rather than bare-bones native installer. If you want to install secureblue on a HTPC or mediaclient device, you cannot use GNOME Remote Desktop (probably neither VNC) for remote control.