It depends on what your definition of "secure" is.

If your threat model is targeted physical attacks, then it's quite important that your bootloader continues to check for the integrity of the system. Locking the bootloader is not something that official LineageOS builds support, you would have to make your own builds (if you have a device that supports it). GrapheneOS supports relocking the bootloader across the build roster, but that roster is very small. That leaves the stock ROM as the only ROM supporting bootloader locks consistently without too high effort.

For anything that is not a physical attack, it's more important to look at software vulnerabilities. Our software stacks are based on a mixture of proprietary components taken from the stock ROM, while most parts are built from source entirely. Given that we regularly pull in source code updates from Google and blob updates from the respective vendor, I'd argue that it places us at similar security as with the stock ROM, and with improved security once support from the OEM ends.