I also have noticed the drop in Stun rates, but with new heroes i am getting alot higher damage, its a Seesaw effect.
With the tactical changes Tynon made, the stun rates had to be altered slightly to allow the support hero and new stuns to be effective (maybe not as low as they are though).
Imagine 4 stunners and leader fighting... chances are the enemy may never get hit off... thats kinda annoying and disheartening to opposing player.