Yes, the cycle times are bad and some ecosystems and tasks are a real pain still.
I also agree with you when it comes to the task of auditing every line of Nix code that factors into a given system. Nix doesn't really make things easier there.
The benefit I'm seeing really comes from composition making it easier to share and direct auditing effort.
All of the tricky code that's hard to audit should be relied on and audited by lots of people, while as a result the actual recipe to put together some specific package or service should be easier to audit.
Additionally, I think looking at diffs that represent changes to the system vs reasoning about the effects of changes made through imperative commands that can affect arbitrary parts of the system has similar efficiency gains.