They say it won't work since the power levels and techniques are different, so there will never be a mutual understanding. Some people may argue that other universes have a chakra system, while the other side say that they don't; thus not even getting into the actual argument.
Of course, it's not like the VS section we have now is riddled with debaters hugging each other at the end of VS match. So supporters could argue it won't be much of a difference than there is now, there's very little debates that members actually agree on. A lot of them go on forever. Then there's the argument that there's numerous sites that allow crossovers. However, the mods could say this is NB, and sometimes NB can make a complete mess out of simple things. I mean I have seen many debaters use the ol' "Oh gawd, NB debaters lololol". Even the debaters imply that the members of other sites are more mature. Members who are on OPB also say that there's not that many trolls or drama there.
Though these matches could work well if they're setup, with judges and all (not allowed to do a contest though).