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.
