I can remember back from my more experimental days, the moment your bootloader is unlocked, your phone is wide open to a lot of mischief and attacks. Meaning if you have to leave your bootloader unlocked to use a custom ROM, it’s probably not ideal.

On the other hand, your manufacturers OS might have some intentional backdoors, or just insecurities in some of the OEM apps, that make it easier to exploit, but that’s obviously hard to say/check.

From my quick search, it looks like it might be possible to re-lock your bootloader with certain manufacturers of you have the right signing keys, but that’s definitely some advanced level fingerling at that point: https://www.reddit.com/r/LineageOS/comments/n7yo7u/a_discussion_about_bootloader_lockingunlocking/

Well, they say they’re studying at a university in Slovenia, which gives it a pretty good chance that English isn’t their native language. Those questions still get across what they’re asking, so unless you’re some kind of grammatical god, I don’t think you can really complain too much