Beasts of chaos are what you might call a finesse army.
As alluded to above you won't win in a straight grind vs another opponent.
Boardwide low bravery, armour saves and an average to hit of 4+, no magic support, few reroll 'something' buffs arnt going to win most match ups in a straight fight.
We do however play the missions / scenarios very well.
Access to plentiful cheap really fast chaff/scoring units, slow heavy hitters (warherd units), balanced summoning, some dirty tzeentch units, big distraction monsters, one of the best endless spells (which allows you to gang up with you "weak" units first)
And again like above, if pure beasts doesn't gel with you, you can run them as any of the 4 chaos gods (battalion dependant) so it's more like 5 armies in one.