I wouldn't say it's mostly BS, it's mostly common sense stuff that distros should have done already.
I don't know about the Ubuntu CIS image but I had to go through the whole CIS PDF for a job once, and implement it all with Ansible on RHEL. I can guarantee that it makes useful changes, and it truly makes a difference to how you use the system.
But in general this type of hardening is mostly used to fulfill some contract, and it's designed around how Linux was used 20 years ago.
My personal preference is to 1) treat linux servers as appliances and stop letting people login, 2) use containers, MACs, MCS and other such isolation tailored for specific services, 3) network ACL and segmentation up the wazoo, 4) MFA access control and 5) encrypt all the things.