It's the other way around, for every person that actually thinks KGs or Fians are in a good state there are 10X as many who would say they are broken and need fixing. Source: YouTube and Reddit and Steam upvotes/comments
Por que no los dos?
You can both nerf the strongest (Khan's Guard, ranged in general) AND buff the weakest (spearmen, pikemen, infantry in general) while leaving all the balanced units' equipment/skills the same.
That would create an actually balanced, tactically interesting game where all units have specialties and weaknesses. It would make the game balanced in the least amount of changes.
"Just buff everything else" would be a huge amount of work Taleworlds probably isn't interested in taking on, and it would make the chance of them making more new balance mistakes much higher with every extra unnecessary change they have to make. There is no real reason why Khan's Guard can't be nerfed except that some people just have an illogical dislike of nerfs.