I think it makes a lot of sense, as a pseudo-horde faction, that Legions of Nagash is meant to make its money off fighting. That's how a faction like the Beastmen plays, for example. However, the Beastmen do not pay upkeep for their troops. So while the Beastmen are low income in terms of their buildings, it plays well since there's not much beyond very overpriced buildings to spend that money. The Tomb Kings, while not a horde faction, have a very similar feel. Both factions are balanced by army and unit caps (and rather expensive buildings, to a lesser extent) to keep the gameplay interesting. The Vampire Coast, on the other hand, make quite a bit more money through their buildings and faction mechanics even with their pseudo-horde nature. As such, they do have to pay upkeep for their troops, but they also don't have army or unit caps.

The Legions of Nagash, for some reason, have the economic limitations of the Beastmen and Tomb Kings without the compensation - no unit upkeep. They're this weird kind of cross between Vampire Coast (pseudo-horde designed to fight a lot to make money) and Beastmen/Tomb Kings (low income with army+unit caps) with none of the upsides of either (no free upkeep, no Coves, no treasure rite, etc).

With the army and unit caps, I think the direction to go is dropping unit upkeep and lean more on being similar to the Tomb Kings and Beastmen. You might then have to drop income from buildings, but perhaps the Legions of Nagash don't profitably own enough settlements for that to be an issue since Nagash has to do the rite for all the provinces without a special building allowing them to grow.