I assume you’re using it with bootloader unlocked. The issue is somehow some malware injects your phone and roots it, it can just install itself as a system service and just live there and you’ll never know. The security feature that protects against it is disabled once bootloader is unlocked.
Also I’ve heard that LineageOS has not all security updates present since some firmware updates needs to be provided by the specific manufacturer. For Pixel, Google provides it and GOS uses it.
They are very security fovused and is against f-droid’s poor security practices. They do push accrescent through their stores tho