I don't see anything wrong with the inclusion of an Underworld right now...
Assuming it was done roughly the same way as before:
-unplanned work for organizers
-lack of proven activity, so probably lots of byes in the tournament
-balance issues, where do you set the starting vault for them, when do you add them
-reduces number of strong sub choices
-major event/rule change added hastily midway, would have likely affected signups/auction strategy
-time/logistics issues making several of the above worse
Not the worst idea in the world but there's a lot of things to consider.