Could you explain how the ability to unlock a bootloader makes a device less usable?
I think they are referring to that LineageOS by and large (there are probably exceptions) does not have support for relocking the bootloader. Some apps refuse to work with an unlocked bootloader (but there are ways around it).
GrapheneOS has even less usability on my Oneplus 7T Pro, in fact it has none
i'm fine with that "less security" as my threat model does not include crossing the US border.
You're conflating local data extraction with security vulnerabilities remotely exploitable via WhatsApp or RCE du jour. Don't.
It has drastically lower privacy and security. That includes apps getting far more data, VPN leaks, many serious privacy and security vulnerabilities being unpatched for many more months or years and not having greatly improved exploit protections mainly protected against apps and remote attacks. Protecting against data extraction is a small part of where security matters and there's much more to privacy than protecting against security vulnerabilities.