OP surely does exaggerate a little and doesn't bother containing his emotions too much, but he's right in general. At current state Sturgia only has 2 top-tier units that are 2nd best in their class (Heavy Axeman and Heroic Line Breaker), a mediocre noble melee cav (used to be auite good when melee cav in general was good), a mediocre light cav (same here, jav-cav has issues allover Calradia), 2 units that are complete trash and no unit that really excells at anything. This seems mildly disappointing but not a disaster. What makes sturgian situation worse is their low- and mid-tier troops - way under-armored.
I don't really like all those nord-viking-vaegir conversions modders do - they're all about huscarls instead of cav, valkiries, poorly picked slavic looking names, misspelled norse ones and the general idea of an attempt to make sturgians a northern Mary Sue. But I do usually edit the spnpccharasters.xml in SandBoxCore to make sturgian troops suck just slightly less without deriving from their vanilla doctrine too far. First thing I do is, of course, swap the retarded condom-plated helmet (please TW, remove this atrocity from the game completely, not only it makes reenactor's eyes bleed, it's been a laughing stock from day 1) of the heavy axeman for the goggled sea-raider one so they'd look like badasses they are. Then I'd probably give em their coat-of-plates back - these guys should Tank, maybe 10-20 points of one-handed wouldn't hurt as well, bacause 2nd best line infantry should actually be the best - with legionaries just half a step behind. Heavy spearman could probably be good if thrusting polearms were a viable thing in Bannerlord - they should deal way more damage and thrust faster. Until that's a thing I'd probably just give em the ol' war razors back - but that's kinda op, and we, sturgians, don't like to be happy, plus the shock troop niche gets kinda crowded, so this one's questionable. Veteran bowman gets 2 stacks of piercing arrows and either a simple bow (which is just a wee bit better than the norse one) or the simplest longbow possible - that still makes him worse than any other top-tier archer but the gap is a lot thinner. Warriors and soldiers get slightly better armor. Recruits too, just a tiny bit, and I make scythes about twice as common among them just so those battanain volunteers with 2-handed hammers wouldn't feel that great. Think I'd also give line-breakers (both versions) some pelts just so they'd excell instead of being closely behind menavs. A year ago (1.5.7 or so) varyag vets would also lose to sea raider chiefs which I found kinda frustrating since the former is basically a civilised version of the latter and that leadership perk kind of felt like a waste, so I balanced the vets slightly to be just on par with raiders. Think, I did that by tinkering with 1-h and throwing, maybe shoulder armor for the vets, but I'm not sure they are still worse than raiders these days.
The above, I think, is just the right amount of buff sturgian troop tree needs. Other thing is their cultural trait. It's good to see TW finally got concerned and removed the infamous "20% less penalty from snow" but maybe they got it wrong, it was actually a good trait, just not good enough. Make it 50% instead - like the battanian one with forests. Or even 70% - snow is a seasonal thing afterall.