Cargo/Go/npm, as you say, will try to "shade" conflicting versions, which is basically a gamble. It may or may not work at runtime because data exchanged between different nodes in the tree of dependencies may have conflicts that make them incompatible... so having module A depend on version 1 of X, while module B depends on version 2 of X, and then A and B exchange objects that depend on which version of X they use will cause issues... this will blow up at compile time in Rust, at least, if you expose X in the API of A and B... but even if you don't, it's still possible for a conflict to occur if the exchanged information is in the form of serialized data or generic collections.
Pub selects one version of each library. It has great tools like listing the currently latest versions of every dependency, or the currently updatable ones given your version constraints, it addresses Dart SDK bounds of each package and it just works greeat. Can you give an example of when it didn't work for you?
EDIT : in the Lisp world, the biggest package manager is Quicklisp[1] as far as I know... and it works decently! I think Ultralisp[2] might be somehow better because it updates indexes faster or something.