I experienced a bug in Cesium:
var xys = Transforms.iau2006XysData.computeXysRadians(dayTT, secondTT, xysScratch);
var xys = Transforms.Iau2006XysData.computeXysRadians(dayTT, secondTT, xysScratch);
Somehow the url property for defining eop and iau files for me is not working. However, the data option works fine with eop parameters. Are you planning to introduce this option to the iau values as well?