Varyags aren't better units than legionaries or sergeants. Legionaries have better armor and a mace option, sergeants have a mace option and can get throwing weapons the legionaries can't(pilas suck). Both factions also have shieldless infantry of their own in the form of voulgiers and menavliatoi, who have an additional edge against cavalry(OR WOULD HAVE, IF THE AI KNEW BOTHERED TO TRY AND FIGHT CAVALRYMEN AT ALL)
That's two out of five further factions whose infantry can at least go toe-to-toe with the Sturgians. It's true the Khuzaits, Aserai, and Battanians basically can't; Khuzaits and Aserai just aren't infantry factions, so that's fine. Battania is a bit undertuned in general and just needs to get some of its options buffed.
And really, if your team's archers can't figure out a way to commit crossfire tactics or fire from elevated points, they deserve to waste their dumb ammo against the shield wall they're up against.