I want to dig into this, but I'm going to try to be brief because I can honestly go on about armor for about six hundred more paragraphs.
"Wait what actually is going on there?" is a failure.
Most of the time, yeah. I think it's fair to point out that sometimes armor doesn't quite make sense to people that don't understand it, so you have to allow that maybe something could be correct and still look weird. But more to your point, if your eye can't even figure out what's going on, forget about whether it's right or not, that's a big problem. That's definitely a problem for Jaeger. I, a person that knows a lot about armor, can't even tell what they were actually going for. The only thing I can figure out (a gambeson halter top over a mail arming coat that's been made wrong?) is dumb as shit.
Jaeger's shoulder pads: Metal? Iron? It's grey but not as shiny as his clearly metal forearm armour. But it's sculpted like leather, with weird seam line details or something at the edges, with no metal rivets and only a tiny bit of texture that could be intended as a hammered metal effect.
Yeah, man. It's crazy how bad that design is.
Honestly, I'm not a huge fan of big pauldrons on most figures anyway. Historically, that's a very specific thing and I just find it so fucking goofy when some dude that's not even wearing pants or a shirt has giant fuck-off pauldrons that put to shame the most elaborate 16th century jousting armor. What are they even supposed to be attached to? As a guy that likes real armor, 'assembly' is kind of the first thing that strikes me about a lot of fantasy designs; just tell me how this stuff all attaches to each other or the wearer. The 4H are so bad for giant pauldrons that are, apparently, riveted directly to a person's fucking nervous system.
The thinness doesn't bother me too much
I don't think it's so much that it's thin, but that it's so much thinner than the upper torso. It makes them look weird together. Maybe if the design on the abdomen was carried up onto the chest, it wouldn't be so bad. But since they're totally different designs, for some reason, it calls to attention how skinny and 'not very armor like' the abdomen looks in comparison to the bigger armored upper body.
Nashorn's armour bothers me for some of the same reasons, I get that it leans into a very exaggerated fantasy design but their figures are so realistically proportioned and painted that they can't really get away with such unrealistic and weird armour. I mean, are his shoulder pads metal? What metal is any of his armour made of? Is the red shit paint, or some magical glow, rust, or what? His metal horns are hammered super rough with harsh angles but his shinguards are extremely intricate and cleanly designed. Then the random grey chestplate doesn't match any of his other armour in colour or design. He just looks incoherent and I don't really know what they were aiming for.
I have a lot less problem with Nashorn's armor pieces, but I agree that they don't necessarily jive -together-. The chest armor has this like.. chitinous/organic look to it. But as you said, it doesn't jive with the rest of what he's wearing. I feel like he'd look a lot better if all his armor pieces looked the same as his chest armor. MAYBE it would also just look better if his chest armor were the same color as his vambraces and greaves.
I don't know what you mean by the red, though. I'm colorblind. What on him is red besides the loincloth?
I like the horns, though. I don't think they need to match the rest of the figure considering they are essentially just a purely decorative element specifically designed (in-universe, I mean) to stand out and look unnerving/unnatural.