I think Mathematica is HUGE. Axiom/FriCAS does not have every thing that Mathematica have(specially statistics).
Overall it is not that polished UI wise. I think it is pretty good for algebra and symbolic computation.
Let me quote from the book/guide of FriCas:
"
FriCAS provides state-of-the-art algebraic machinery to handle your most advanced symbolic problems.
For example, FriCAS’s integrator gives you the answer when an answer exists. If one does not, it
provides a proof that there is no answer. Integration is just one of a multitude of symbolic operations
that FriCAS provides.
"
You can find out more by reading the book which is available here: