I've been playing a bunch with poly lately given the recent attention in Congress...
It's possible to grab MUC koa quite easily through ruins and then XP this into very powerful early weapon -- with survivalism into the heal every turn and ++healing promos it can quite capably take out forward settled island cities, solo -- I used one galley to block reinforcements and add a few hits at the end, but otherwise experienced survival/heal koa did all the work against Inca that tried to grab a spot, cut off on Peninsula behind my capital. And Inca went through 2 uu defenders too, that could only plink off maybe 6-7 hp on the Koa.
Its possible this is more a problem with the recon tree, recon xp, and ruin upgrades all combining in a way that needs fixing, but it is FUN, so we might instead say koa is a little over tuned for this scenario, maybe you consider something like -1 CS but +10% strength in a very situational application -- it's too bad we can't do bonus when adjacent to water only