I noticed two problems with using Xaddress. 
The first is that at least some number-word-word combination occur in more than one country-region; they are not unique. Related is that the country and region seem to be required, so the minimum length of the address is five items instead of three. 
The second one is that the Xaddress addresses are accurate only to four decimal places, which is a lot coarser than what3words's 3 m x 3 m grid squares.