Yes at the moment they are overpowered,not only in sieges,but everywhere,since they can kill as many as 15 enemies in one shot,and that shouldn't happen.But it's getting fixed in the next version.While an army full of handgonners and cannoneers+rocket projector spammers should dominate any battle because of all that firepower,their costs need to go up and their AOE damage down.
Rhulgs are not overpowered,they are just exceptionally good at what they're doing,and they counter that silly charge command that was soo easy to win with in native.Order your cavalry to charge them,see if they get away with it.Their shields are awful on the other hand,and if you are good enough you can get rid of the enemy ones yourself,then laugh at how the enemy army fails miserably at killing your guys.