The Yathsou is arguably the most powerful ship-to-ship combat unit in the game, being affordable enough to mass-produce in sufficient numbers to overwhelm more powerful vessels like the experimental Atlantis, Tempest or Megalith and powerful enough to dominate cheaper T2 submarine hunters such as the Vesper or Barracuda...it is even a perfect counter for experimentals trying to cross water. All in all, the Yathsou is probably the reason the Seraphim lack an experimental naval unit.