Here's Skirmish retinue totally not being overpowered, with absolutely no traits or terrain in my favour. If you can assuredly rout an enemy in 3 days by stacking every archer on one flank, you win so hard it ain't even funny. This is those same merc groups, just here to skirmish and pin the enemy while archers kick ass.
BTW, if there is one thing Republics are ridiculous for, it's ships. I have almost no coastline but these guys are supplying me with 80-150 ships off at most a duchy worth of provinces. They seem to not do good economically this early on though because AI can't into tech rushing Trade Practices.