The lest discussed or avoided question about security of GrapheneOS is the openness of software:
Drivers and firmware are closed source, from Google. Do you trust google?
How do grapheneOS developers verify if cellular modem does not call google every 10 seconds sending GPS data and imei? You would need grey cell tower to intercept cellular modem traffic to analyze. Did they do it? I'm not sure
Do you think closed source video, wifi drivers and modem firmware are free from backdoors or bugs? Is iommu isolation of devices used by grapheneOS sufficient to prevent sending of screenshots (graphical information processed by GPU) through cellular modem - when they are located on one same physical SOC?
How many backdoors are left in android? Why pegassus was so successful?
I would trust more linux based solutions - at least whole world runs linux servers and they are not that easily breakable if you know what you are doing.
Beauty of grapheneOS is that it is user friendly and requires little user involvement for decent out of the box security and privacy
LUKS provides industry grade encryption. You have freedom to use more secure algorithms, e.g. cryptsetup -v --key-size 512 --hash sha512 luksFormat ... that you can't do on GrapheneOS
You can encrypt /boot to limit access to kernel; leaving only bootloader as vulnerable part
You can use sha sum verification of packages and files to monitor intruder modifications:
https://wiki.gentoo.org/wiki/Security_Ha..._detection
Without secure boot you'll be vulnerable to physical attack, when somebody get hold of your phone - then bootloader can be modified to use malicious kernel, or steal you encryption password - when you boot up your modified system. But not sure how easy this can be done
W/O encryption password nobody will get your files. But this means password should be long and secure
Physical access to device also means GrapheneOS pixel phone can be physically modified - so no security at that part neither
And secure boot won't help if somebody got already remote access to your device, this is game over
on linux easy to use isolation of apps is through flatpak
A bit better solution is firejail, but this might require tinkering
But linux phylosophy is different to android: user usually do not install programs with closed source code or from untrusted sources. So impact of malicious apps on general security is low on linux.
Moreover, app isolation does not help on android with malicious apps exploiting undocumented vulnerabilities - as with pegassus