The main problem I'd imagine with making a cross DC PF is the NA and Japan servers... They have 4 DCs, which are all essentially separate server machines, and they don't have your characters data just stored on all the DCs, that data being sent over is part of the DC travel.

In order to sync up with all the DCs, you would effectively have to go through the process of traveling to every DC, including any wait times and slowdown caused by multiple people doing it, and to ensure the data is properly up to date, it'd need to be done once per login to ensure it's all up to date.

On Europe with just 2 DCs it would be less of an issue, and Oceania can't even travel... NA and Japan with their 4 DCs, would take considerably longer.