Undead/Sage, It's actually really solid.
First, I'll discuss it's strengths. It goes without saying that undead is one of the premiere attacker races. Their access to plague and plague immunity, their attacking casualties reduction, and 11/3 elites, make them an attractive race to attackers and warring kingdoms age in and age out.
However, it is their lack of need of food which I want to talk about. This lack of need for farms frees up space in your build strat for buildings which benefit sage, labs and later universities. The undead's starvation immunity also compliments the sage persona by allowing undead/sage to forgo development of food sciences (this allows you gain more scientist in the other, arguably more important sciences).
In the same way that the undead race compliments the sage personality, the sage persona compliments the undead race. Sage bolsters and accelerates science development and effectiveness- which allows you to gain greater ME more quickly, reduces to eliminates the need for banks more quickly, and coupled with homes you can accrue quite a massive population rather quickly. I
I've ran the combo for little more than a week now and I've already been able to eliminate the need for both farms and banks.
The laboratories are working a lot better than last age. I would also say that the new science system is working a lot better this age than the last.
I'll shy away from discussing the detials of my build strats because players hold different views of economics but I'd like to mention afew things about buildings in the strat. In the past when I would sage, I would keep a small science pump constantly running and I would bolster my science defense and development with schools, and a minor bit of extra SE bonus from libraries. Now I'm running a decent amount of labs but as my prov develops I will phase them down quite a bit and start splashing some universities into my start.
In wars, I'll drop out all labs in favor of more of the standard attacker buildings- barracks, training grounds, hospitals, etc.
The obvious weakness of the strat is the undead's inability to rob vaults, coupled with the cost of ghouls, slows their development somewhat. This makes fortifying undead's favorite stance. Fortify doesn't help a developing sage though- your scientist will not progress towards professorship and because scientist will not develop while in fort. Laboratories are practically useless in fort. Therefore, as undead/sage, you'll both love and hate fortifying.
Do I consider undead the most powerful variety of sage? No, to my mind the most powerful sages would be dwarf/sage and halfling sage. However, I do consider the undead variety of sage the most interesting for the reasons I mentioned.