As far as I can see, the Underworld team as it is is a mixture of the lesser parts of two other teams. I mean, they lack the gutter runners that give skaven their edge and on top of that they only have one troll and no secret weapons from the goblin team roster!
I have seen Underworld teams in action before and I would never ever judge taking away their skaven players in order for them to become stunty teams. I have seen full squad Underworld teams being played by good coaches and getting soundly trashed by halfling teams!
The only team I could honestly come up with as a worse team than a skaven-less Underworld team would be an all snots team. And that even has me doubting ...
Therefore we actually allow Underworld teams as stunty teams, period. Both for the Belgian Championship as well as for the Belgian Challenger Cup. I'll come back on this topic after the Belgian Championship (though I doubt there will be anybody crazy enough to come and play Underworld) and after the Goblin Slaughterfest (where we do normally see a lot of stunty teams and who adapted the same rules as the BC and BCC ...).
I'm a big fan of all-skink teams, but also doubt that they are in the same league as the other stunties such as goblins, halflings and ogres. You might be able to do a lot more with the skinky boys than with the other ones. Losing the throw team-mate option is there, admitted, but you do get a lot of players that are a LOT faster than your average stunty player!
Would have loved to see the face of the guy that faced the dark elf team with Horkon and two assassins!!!

Cheers!