I think the wiki page for `location=*` provides a reasonable guideline for when to use the `tunnel=*` key.

https://wiki.openstreetmap.org/wiki/Key:location

> "For features which are contained within a _self-supporting_ tunnel (...) use `tunnel=*` (...)"

This distinction is quite reasonable I think. If a cable is simply buried in the ground without anything around it it would just be tagged with `location=underground` without a `tunnel=*` tag.