For many projects you won't need that complexity. You just want to directly map basic matrix operations to the usual BLAS/LAPACK calls. Fortran 90+ does that job well, for instance, and performance will usually be better than a C++ library (yes, I've tested against Eigen and Armadillo, although that was years ago). Combine that with the enormous compile times and the absolute ridiculous error messages for even simplest syntax errors, and I never looked back. Fortran may have a bad rep, but the newer iterations are actually pretty good for that kind of stuff.