src/Distribution/Nixpkgs/Haskell/FromCabal/PostProcess.hs has a lot of post processing hooks which do things that would also be possible via overrides, like disabling tests, adding dependencies, adding a phase hook etc.

I've stumbled over this before since cabal2nix sometimes does “magic” stuff I didn't expect. To me it'd be clearer to move these changes into nixpkgs's configuration-*.nix files, but there is the risk of breaking stuff for users of callCabal2nix, callHackageDirect etc.