That is because STAB multiplies the power of a move by 1.5x, which is an increase of 50%.
Mooguru is part flying type. He would get STAB from flying attacks. Brave Bird is a 120 powered flying move. Even if the power increase was by 10% the move would be 132 power. The STAB increase would bring it to 198 Power. It is still over powered.
Hihidaruma is pure fire. He would get STAB from fire attacks. Fire Punch is 75 Power. If Encourage was 10% of an increase it would be raised to 82.5 Power. If you add the STAB Increase you would get 123.75 Power. It is still going to be over powered even if it is as low as 10% so long as you have STAB.
Well... Brave Bird has 120 Power and with STAB is 180. So already it's kinda over powered.
BUT in my opinion, I think that this ability will have another drawback, I think it only powers up moves that HAVE a secondary effect.
Hmmm... Like a trade. If you want extra power you need to give up your secondary effect. No secondary effect? No extra power! BYE!
I think ONLY having it negate secondary effects is not enough.
As for how much it raises, I'm going to say 25%