I have tried to use the new BLAS/LAPACK infrastructure to build R with MKL and the resulting R produces incorrect results for matrix multiplication (see my comment Add BLAS/LAPACK switching mechanism by matthewbauer · Pull Request #83888 · NixOS/nixpkgs · GitHub). I have opened an issue here R built with MKL computes incorrect results · Issue #104026 · NixOS/nixpkgs · GitHub which has the code I used to build R.