I'll admit these guys are crazy strong if you get enough of them... Move shoot twice and run... Once level 3 they are mini field cannons that move after attacking and ignore hills (or forest)...That's because you haven't massed their UU yet and used the 100% experience recon card. Inca has the best UU in the game. Tied with Toa, arguably better than Pitati.
The Incan Waka Raka makes all other units die in shame.
Main problem is that you have no SP card to increase their production.. And they still die to horse units if you are not careful to abuse terrain... In flatland they are not that good.