After playing for some time with Excalibur Umbra, there are two problems i found regarding his passive:
When going into Operator mode, the AI specter always removes your exalted blade. This is a problem if you play with Energy Conversion.
Not sure if a bug or a feature (i guess it's a bug), but Excalibur Umbra's specter isn't affected by Magus Nourish and Medi Ray.
The first problem also means that, if you want to use Magus Elevate and you are playing with an Exalted Bla