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The main advantage of the Necropolis is that it is a Castle with almost no weaknesses. In the first week, 120-140 skeletons led by Gosh destroy everything in their path; in the second week, a second front opens with upgraded Vampires, and by the middle to end of the second week, with a favorable situation, the number of skeletons (Angels and Ducks are useful, but not as critical as for other Castles) may be sufficient to break through the Undead faction. That is, the Necropolis has almost no pronounced "dips" in its power.
The above applies to playing on the 8x8 map. However, on the small map, the Necropolis may find it more difficult due to the small size of the map and the ease of rushing it. That is, if you get a poor underground resource spawn, you're out of luck. On the 8x8 map, this issue is somewhat more manageable, with more Castles/towns (skeletons), more territory, more distance, and more options to prolong the game (which is almost always in favor of the Necropolis, as skeletons accumulate). Therefore, landing in the underground is not as critical on the 8x8 map as it is on the small map.
I don't particularly enjoy playing as the Necropolis myself, precisely because it is too strong. However, playing against it can be interesting. For example, I have played the Red Flax vs. Blue Necropolis matchup repeatedly, including in tournaments, with varying degrees of success.
All of the above applies to playing on the Total Annihilation map with the Necropolis patch.