It's largely mitigated by allowing multiple versions. Imagine A depends on B and C, which both depend on D. The diamond dependency usually pops up when B and C

It's largely mitigated by allowing multiple versions. Imagine A depends on B and C, which both depend on D. The diamond dependency usually pops up when B and C need different versions of D. But in this case, Nix allows you to simply install another version of B or C such that they can share D. This doesn't help you if there's no set of A ... D that are jointly consistent, but then you would've been SOL anyway.