The campaigns are generally boring because the first ones are always easy... and the developers are to blame for that...
But the faction itself is good, and there's no need to criticize it... and why are the Incas considered weak? Maybe you're the one who's weak? You're saying that without providing any arguments... It's a good unit that fits well into the Haven army, and it has suitable skills... buffer + archer + decent warrior (HP, attack, no penalty)
The only downside of the Incas, and other human units (maybe except for angels), is their graphical design... they were poorly drawn.