Okay...I have played with the Grifori a couple of tiimes and all I have to say is...WOW!!!
In my first game my impression was that they were underpowered in the early game since the lack of religion made happiness and income an issue. I had six or seven adventurers, but I could not upgrade them to anything better than a warrior since I lacked the necessary funds.
In my second game I tried a little different (more aggressive) approach. I rushed for the bronze working technology and upgraded two of my adventurers to axeman and led an invasion force (with about 10 additional axeman escorting) into rival territory early on. I used my adaptive trade to get raiders (extra gold and exp) and went to town. By turn 100 or so my two initial heros had combat V and heroic strength II.
I saved this second game and will continue it later. I have eliminated one AI opponent and I am close to finishing off another. I have upgraded a third adventurer to a ranger to scout out the map (str 8 with mobility II and a hawk). I have two more in reserve, one to make an archmage when the time comes and the other to make a longbowman (very soon).
Grigori have to be the best Civ for conquest. I haven't even built the Baron yet and I already own about 20% of the map (large, monarch). Any weakness by not getting the religion bonuses is more than made up for with the additional heroes.