You should have seen LA Mictlan in my previous game. Their S9W9F9 bless makes my current army look like weak sauce. Though in that game I did at least learn the value of Horrors.
I've played one SP game with Shinuyama and fought a war against them in MP. So I'm not an expert on them, but I have some knowledge. They seem like a well rounded nation, with several good facets but nothing overwhelming. I think their main weakness is in the early game, where they lack the bless power of MA Ermor and the like. From my memory, Kappas are too expensive for expansion, though they do give you the option to enter the water. In the mid game, Shinuyama becomes more deadly as their mages bring out battlefield enchantments (Flaming Arrows, Strength of Giants, Legions of Steel, Weapons of Sharpness) which turn even bandits into mini-thugs. In the late game they have hammers and decent forging options, but aren't really setup to make clams, and don't have the strong Air or Astral magic that you really want in the late game.