Eigen is similar to xtensor, but with 1) Eigen being significantly more mature, and used in production in simulation/research environments and 2) Eigen having not just basic operations on arrays, but also a full suite of linear algebra operations comparable to a full BLAS library, including a full test suite. xtensor wins on having better integration with Python and other languages out of the box. There's also some performance limitations due to being inspired by numpy, particularly with respect to memory management.