I used a thug communion once against the ai in a singleplayer game. I had 25 or so of them go up against 300 various t'ien ch'i troops, including a decent number of chariots. Only lost 3
But I didn't retreat the masters. Do you have to retreat masters in order for them to work properly, or is giving them attack orders good enough? I had a number of them who stood around casting spells for some reason even though they were all scripted to attack at the end of their scripts. If so that might explain it....
Alugran manifest thug communions are certainly fun though, when you're not fighting against the most powerul astral nation to have ever existed :P