So: In case anyone's still reading this, here's my thoughts:
First the big thing: I had no idea the kind of early game power I was sitting on with Niefelhiem. Niefel giants are crazy. This didn't really click until I sent them against Ermor and he tried to beat them with unsupported legionnaires. Geez. I should have been expanding with groups of nothing but 4 Niefel giants + a Gode. I also didn't understand how to use magic, which makes sense because Neifelhiem is actually kinda weird with magic. They have good path access on paper, probably one of the best factions for construction (Niefel Jarls can become light SCs at construction 4 and maybe a few alteration levels), and decent blood access. They can also make ice fiends, which is a bit redundant but hey, its a good spammable unit and it doesn't directly cost gold so of course Niefelhiem is all over that.
That being said, Niefelhiem sucks at using their mages for combat support. This is something I didn't understand for most of this game. If you use Gynjas, you're paying for 3 paths you aren't using, and also for all the Gynja that randomed things that don't fit into your plan. If you use Skratti or Niefel Jarls, your paths are great for self-buffing but mediocre at everything else, and also paying for a bunch of combat stats. This pushes Niefelhiem into using thugs. The other issue Niefelhiem has is that they just don't have enough dudes. Jotuns aren't actually good fighters in the scheme of things, they have poor per-tile power and in the EA you don't get any national troops that can share a tile with them. So you use the Niefel Giants, but then you're paying 175 gold EACH, and unfortunately for me my pretender design I went dormant for some reason and then didn't take the proper scales for gold. So there just aren't enough dudes to be able to split armies effectively, further making you reliant on thugs with Gynja for research/construction/rituals. Coming into this game, I didn't know how to use or make thugs. I only learned about 6 turns before I tried to attack Pan, and at that point it was clear that no matter what i did, if Fomoria or Ulm decided to attack me in the back I was going to collapse. Long story short, Ulm did, and the rest was history.
Lastly, I got unlucky with terrain this game (not that I was playing well enough to win either way). I was pretty efficient about site searching for a new player, and I even got heroes so that I could search every single path at at least 1. I found like two gem gen sites total in my territory in the first 18 turns. Instead I got a bunch of crappy "mundane" sites like supply generators, extra recruitment, extra resources. The only useful one out of the lot of them was a library that let me recruit sages, which was... actually pretty amazing. Basically negated my nation's research disadvantage, not that I knew what to do with the extra research.
My neighbors were Ermor, Pan, C'tis, and Ulm. However, my "natural" opponent was Ulm, and there happens to be a guide on the internet about how to fight Niefelhiem to a standstill as Ulm by taking advantage of two disadvantages: poor per-tile offense (which Ulm's dual wielders are good at) and fire vulnerability. Given what I know of Hellheart he would have almost certainly crippled me if I'd tried to go after him at my skill level back then. The terrain just wasn't good for fighting the other three. I could have negated this disadvantage by, um, realizing that Niefelhiem is insane and just picking one person and rekking them. I allowed myself to be discouraged by seeing other people's 100+ armies and thinking "shit, all I've got are these 20 giants". I also should have fucked everything else and researched construction 4 so I could turn out SCs unreasonably early. Instead of going after fricken conjuration because that was the only way I could figure out how to make my mages do anything useful.
What really, truly wasn't fair about the terrain in this game was the fact that it wasn't double wrap. Pan had, what, 40 provinces? Only 6 of which were bordered by other land nations. That's a whole lot of resources being channeled into two very narrow chokepoints. The last wars saw Pan and Ulm fighting against Fomoria and Neifelhiem, which doesn't make any sense politically. Ulm was close to winning by thrones, Pan was close to snowballing to the point where they could fight everyone at once. It should have been them duking it out and us two picking sides. But because of the one way wrap, it was almost literally impossible for them NOT to be de facto allies against me and Fomoria. They both got a big advantage by having a long safe border at the top and bottom of the map. Again, not that I would have won either way.