Its one of those wow-factor weapons, like the blade of realities. Your opponents will be sh*t scared of it, and will try to avoid it at all costs, which will make it a lot harder to get into an effective position. Also remember if something faster than the steg (all cavalry, fliers, maybe chariots) decide to go for it, all they need to do is kill the t2 chief off the back and you lose a lot of points. It is a glass hammer, I have opted to drop it in my lists at least until 3k or so but it can be effective. Try it and see how it works for you.
On the steg vs ancient, I really don't think the ancient is worth the upgrade points for the chief.. If I am taking an ancient, it will have a priest with engine on it. If they were going to give the ancient a couple of stats worse than the normal steg, they should have made it a lot closer to the same points if not the same. Either that or keep as is and don't give the ancient any downgrades in stats.