First things first, this is a good paint job. There is a lot to like about the brush work and a lot to admire overall. It's not without its faults and not in the same league as some of the models on CMoN, but you've done a great job overall and should be satisfied without a doubt.
That said, you've asked for a more in depth critique, so with no extreme niceness pushed through and perhaps some slightly blunt honest opinion (and I'm by no means correct of course, but hopefully I spot some useful things) here we go!
The standard response is that scores don't matter. It's true, but even though I say that, I can't help but care about and question the scoring here sometimes. As far as that aspect of things goes, try not to worry about the score too much. Is it being underscored compared to the quality of other models in the same scoring range? Probably. Are there reasons for that? Probably. Are those reasons to do with other factors outside of painting as well as the actual job you've done on the model? Probably!
The first thing that's very clear is that your photos are honest as honest can be. That's going to help me give you more in depth feedback, but it's also showing off a warts and all look at your model. What I mean by that is that the views of the model are large, so there's nowhere to hide. That last close up is bold and shows that you've done a good job with some of your blends and techniques, but it's so big that it's highlighting the flaws way more too. Very few painters are smooth and good enough that they can show an image that large and not have it highlight flaws.
You mention the white background showing it off as well as it could and I think you are correct, it's not doing it any favours and it's robbing the piece of some mood. That could do with being changed for sure.
A final thing as far as presentation goes is that I like is to find detail shots of parts of the model that highlight parts I am proud of. It is an excellent way to improve the presentation and show some close ups without it being too in your face and overpowering.
I believe I saw the really cool cracked mud effect on the base in a WIP thread of yours and I'd have done a detail shot of that for sure. Likewise, something better showing off your glistening gore technique.
I mocked up a really quick, different version of your images. I didn't do all the angles, and had to make details from what was available, but in my opinion, something like this will really help you score more if that's what you are after. The flies were clipped out because the wing membranes just looked odd showing the white background through!
I see this sort of presentation part as another element of showing off your model. You've put in a good bit of time to make it as presentable as possible in the flesh, so there's no reason you shouldn't do the same with the photo montage. Make sure it shows the model off as well as it can.
See how it's smaller, tighter, moodier, but still shows off the model and cleans it up? I may actually have gone a bit too small with the image width, but I wanted to emphasise the point.
Another thing to consider that could be hurting your scoring is that you've used a model everyone has seen an awful lot of recently. Perhaps not a biggie, but things like that do make a difference.
OK, so that's a whole lot of gibbering about stuff without talking about the work you've actually done on the model, so I'd best get onto that!
The concept is a cool one, and the execution of it isn't bad either. The conversion work you've done to remove the axe, tidy up the stomach and fix the hands looks clean and seamless, so that's great. The one thing that's not quite as good are the wings on the flies. The shape is just too angular and this makes it look a little less organic than I'd like. It's ok mind you, but could be improved. I'm less keen on the scattered body parts on the base. They don't make a whole lot of sense to me. What has chopped them up like that, how did they get there and why are they so gory? It doesn't enforce the Lord of the Flies feel and there's not as strong a narrative flow behind the model because of them. More solid corpses, rotting, covered in a load of flies would have worked so much better.
The base itself is probably a little bit too big and a little bit too one dimensional. The flat, cracked earth is groovy and has been done well, but if you are going to keep it barren and sparse, I think you should have also made it almost half the size it is. Right now the main model feels marooned in the scene a bit. Also, the few tufts and lone rock feel very tacked on. Something more in theme with the cracked earth, like interesting shaped, wind ravaged red rocks and perhaps some cacti would have added a lot more interest.
Quite a lot of 'negatives' there, but please look at it as areas for improvement. Like I said at the start, I like it, but I'm really over critiquing it.
So, the painting on the Nurgle Lord.
First off, the armour - I really like the boots. Perhaps they could be pushed just a bit more in shadow and highlight, with that final extra sharp contrast in places, but the overall look is an effective and quality dirty NMM. The chainmail hanging down is less effective. I think the problem is it's been over weathered, so the links don't have quite enough metal coming through on the edges, where rust would potentially be scraped and shined up again. Also, on the back, where the mail shows through the hanging fabric, I feel those areas need a lot more shadow and a bit of weathering.
The shoulder pad is pretty good too, but again could do with a wee bit more contrast and some more sharp edge highlights. The helm doesn't look quite as neatly done, but it's still got the same nice tonal quality of the rest. The blood splat on the helm is a bit too big for me. I'm thinking that you've tried to draw the eye up to the face as a focal point by replicating the two colours that crest under it (the arc of blood from the hand to belly to hand, and the arc of glowing green in the three wounds) which is a good plan, but just a bit more subtlety on the gore here and indeed the glow of the eyes too would help. Those under highlights on the ridges of the eye slots are too thick and lack refinement. I totally respect the way you've used colour to try and bring the attention up though. That's darn tootin.
The skin aint bad and the colours in the folds are nice, but it could be smoother. The shading line under his moobs is particularly severe. The biggest issue here though is too much uncontrolled gore. I think that you'd have done a better job applying a fair bit of splatter with a brush and then giving it a final, much less over the top and all over the model final flourish with a quick random flick. It is too slapdash compared to the effort you've put in elsewhere. While blood flow is quite splattery, it comes in from angles and pumps out in streaks and arcs. Here it's just a dusting all over of spots, which doesn't work as well.
Crikey, I'm doing a fair bit of writing here... best wrap it up.
The legs - I think they are more flesh, but you've done them in a kind of trousery colour, which is a bit odd to my eyes. The fabric is way too clean on the blue bit. Surely that would be the grubbiest part of all, so it needs to be seriously weathered. And the skull needs some final spot highlights to finish it off.
And I guess I'll finish off by saying that I've just given your model the sort of harsh assessment I'd usually only apply to artists in the very top of the painting world, so it's bound to sound fairly harsh. There's a lot to like about your model despite my ranting, so keep up the good work, keep trying to improve and, if you are bothered about scores, you can help yourself out there without changing anything about your painting.
