ARX functions are difficult to cryptanalyze, thus the reason why it
   takes longer to reach the optimal cryptanalytical method against it. In
   the NIST report on SHA-3, it was commented that there is less
   cryptanalysis against non-ARX functions than say Threefish.
   It also shows that the cryptographic community is susceptible to
   publication bias.
   The greatest issue that Keccak (pronounced ketchup) has is that it has
   a lot of side channels... like Rijndael.