So which are the optimizations the Intel compiler does which GCC can't is asked? I could guess at the reason for a factor of two, but what does the detailed profiling say with equivalent compiler flags? I can also say that GCC is a factor of two better on SKX on a Fortran benchmark, and came out about the same over the collection that's from when profile-directed. The usual reason for the Intel compiler appearing to win much is incorrect-by-default maths optimization allowing more vectorization.

I don't know about MKL stability, but reliability definitely isn't something I associate with the Intel Fortran compiler (or MPI) in research computing support.