Why is Mathematica not simplifying further even after providing assumptions?

Question

m = {{0, 0, 1, 0}, {0, 0, 0, 1}, {-5 X^2, X^2, 0, 0}, {X^2, -X^2, 0, 0}};
Simplify[Eigenvalues[m], Assumptions -> X>0]

The output I get is this below. The last two eigenvalues are not in their simplest form. They too are complex numbers.

A little mouse on the pampas · Accepted Answer

I found a simple way:
   Simplify[Eigenvalues[m], Assumptions -> X > 0] // PowerExpand

I think the function ComplexityFunction or function TransformationFunctions is likely to do the same, but I don't know how to do it yet.

One Answer