The original Thermion XG used the BAE sensor. This is the sensor used in the Trijicon and Nvision units. However, they did not use the firmware as they wanted to utilize the traditional Pulsar features. For whatever reason in my opinion and Tom Austin's, the XG never lived up to image of the other two brands or in many situations even the XP line up of their own brand. It appears Pulsar is discontinuing using the BAE sensor and is going back to a ULIS style sensor in the newer XG units.
As far as issues, Tom never mentioned holding zero as an issue but had lots of glitchy issues with his personal XG.