Tenure-track reference letters: previous supervisors versus outsiders
It's a lot harder to have non-advisors write letters for a tenure-track application in most STEM fields, largely because of the more direct connection between projects, funding, and supervision in STEM fields versus humanities fields.

In the applications I've reviewed as a hiring committee member in engineering, pretty much every letter came from either a PI or co-PI of a project the applicant worked on as either a graduate student or a postdoc. If there was a letter from an "external" member, I don't recall seeing it. However, it certainly wouldn't hurt if such letters were available, so long as it's a helpful letter of recommendation.