I have posted frequently on the Discord regarding some of these pain points but with the Discord feedback section now discontinued, I have decided to make a proper forum post looking into the issue at depth for a single player only perspective on Nurgle in general, and a bit on Kugath (who literally is the saving grace of the faction that would not function partially without him).
As a bit of background, I have played Total War since Medieval 1 where I was a young child freshly moved out of London to Kent, spamming peasant stacks everywhere on the easiest difficulty as Egypt or HRE and being utterly confused why I would be losing some battles of 40v10. Since then, I have been a Legendary or VH/VH difficulty player, doing all the campaigns until their natural streamroll lull, pouring in over 3000 hours into WH2, 1000 into WH1 and well into the 10's of thousands of hours across Total War titles.
To give a fair representation to the faction, I will only be using Normal campaign difficulty modifiers. We shouldn't base discussion on a minority factor that may be the cause of its own issues. I will follow this initial part of the post with more, but this should be a nice starting point. So, let's get started!
Part 1 - Unit Cost
Summary: Nurglings are the only cost effective unit, whilst the rest of nurgles roster costs the same or more than their competing factions equivalents for far worse units.
This is the big one, and thus the one I wanted to start with first! It's turn 32, I have 3 full provinces, 1 fully developed, 1 at 4/2/2 and the other at 3/1/1. Public Order isn't an issue, and my income is sitting at 4968 with the one Kugath army. That army is identical to its start but with 2 Plagueridden (one being Death for obvious reasons), and filled otherwise with Nurglings.
This costs 3025 per turn. BUT that is due to a 25% reduction by Kugath, bringing them from 88 to 66. That is 22x10, 220 extra upkeep we should add. So my income is actually 4748 with an army upkeep of 3245, with what in all honesty is almost effective as a skele boi spam army (in many ways, less so if not for Kugath himself buffing them in his army).
These can be recruited at free using the Auxiliary + Kugath's faction bonuses. These allow Nurglings to be your pop-up defensive deterrent, as what I believe they're intended to be. The issue is, this is a Kugath only thing. Unless future factions take his affects and add onto them, this won't be how Nurgle functions. So, for a fair and proper look, we need to remove Kugath's bonuses like I did earlier for the economy summary, in each situation.
So, first we have the insanely high recruitment costs. Plaguebearers sitting at 1125, 189, after the first recruitment reduction technology - compared to 0 recruitment cost and 70 upkeep for nurglings with Kugath bonuses, or at sub 200 recruitment without. It is immediately no longer viable to even recruit tier 2 units. Even just replacing the Nurglings in 1 army at this stage with tier 2 infantry, will bankrupt my campaign. This may seem reasonable when we think of Pink Horrors being tier 2. But are Pink Horrors equal to plaguebearers in effectiveness? No. Even if you gave these guys twice the melee stats, their speed and vulnerability will always make them worst for tier. This is not an acceptable cost for their use.
For reference, a pink horror costs 300 less, for 10 more in upkeep. So a regular plaguebearer is similar to cost to an Exalted Pink Horror. A tier 3 unit that would decimate it.
But then, let's look why we're struggling for economy to pay for these troops!
Part 2 - Nurgle GDP, a case study in third world economics.
Summary: Nurgle low income generation, with no modifiers.
Whilst yes, Nurgles buildings do not require further upgrades to develop, this is a double edge sword and actually doesn't just generate economic issues, but also can trap new or average level players into deficits due to the ebb and flows of their revenue. That 4748 income I mentioned earlier? That is with a carefully laid out development to ensure all my buildings are at their peak at the same time (I used Cheat Engine to do this, as it is practically impossible to do manually otherwise). In reality, my income was on average otherwise in normal play, up to a full 1068 lower at some parts of the ebb. Being actually on average 3680. That is an average income of 1k per province. So, what is the full economic potential of these 3 provinces? Well, let's find out!
Let's breakdown the cost of getting these 3 provinces fully developed from where they are and what income we can expect to see from that (in maximum values, not real values that the player will see - we have to give A LOT of handicaps here). This does mean however building the overall useless fly unit tree, being it is the highest income generator for the faction.
So, let's take a nearby province that isn't too exceptional (doesn't sport 2 ports for example like in Kugath's home province). Path to the East - it has 4 regions, only 1 trade resource, and no unique chains. Perfect! A totally reasonable province that would generally produce 3k income depending on your faction elsewhere (would reach higher, but as a Ogre settlement province you are limited to only 3 slots and no capital city province). What does Nurgle get?
So, the cost to build 1 slot then start upgrading to tier 2 is 25,380. The same process for 1 more slot and tier 3 upgrade is 39,560 (Yup, we've increase the cost 14,000 or over 50%), with the final building slots being basic infrastructure at 9,600.
That is a sum total of 74,540 to fully develop a ogre region. I allowed for 1 defensive building to stop the cancer that is portals, and focused on income rather than practicality (where in honesty, you're better off spamming nurgling chains for basic settlements due to garrisons to reduce overall expenditure on armies).
Our total? 3892 per turn. So we're looking at over 20 turns to see return which isn't bad, especially with unit generation and hero cap increase, especially for an ogre province!... right? Remember this is max values. Let's compare this max unrealistic and hyper inflated value, to the Tzeench income of the same province.
3522, for half the construction cost. That is 300~ lower than the MAXIMUM value of Nurgle, that is consistent value.
Now, this would make Nurgle appear still even at maximum value, the overall loser. It is 200 extra income per turn, for an extra 30,000~ investment, and generates 2 less heroes and less overall faction resources as well.
Let's make that gap wider - let's introduce the minimum and average values.
Average: 2745 (rounded up).
That makes Nurgle 800~ on average poorer, than the birds who are a weaker economic faction in WH3. And potentially at the lowest, less than half their economy. For double the construction cost. And far more expensive recruitment and armies...