The OS needs access to the keys accepted by the bl. Pixels have that, so graphene can relock. Other vendors don’t give this opportunity, so Roms like los which support many devices, better be safe than sorry.
Try relock on a xiaomi device and you have unrecoverable hard brick.
I do. I find it very useful.
Is it safe? No idea, but its read only, so seems that they cannot operate on your bank account in any case. That feels safe enough for me.