Try playing with any decks against a Pharao - Scarab deck and you will see what I mean. Otyughs are not so overpowered because they have not the auto buff ability like scarabs have.
Unlike Lightning, Shockwave and Snipe, Devour is limited in what it can target.
This is significant as you will note the only change between Otyugh and Elite Otyugh is the hp increases from 3 to 5.
So
Devour can destroy creatures of less than 3|5 hp for 1 
Snipe can destroy creatures of 3X hp for 2X 
Lightning can destroy creatures of 5X hp for 2X
+ X cards
Snipe costs 2
. That is double the amount of a devour. And you can not kill creatures with one hit if they have 4 or more HP.
Lightning costs 2
and a card. Devour costs 1
and no card. Furthermore is the devouring creature gaining an increase in attack and HP.
The "X"s were there for a reason.
Devour can destroy creatures with less hp for 1

Snipe can kill creatures with hp 1-3 for 2

, 4-6 for 4

or 7-9 for 6

Lightning can kill creatures with hp 1-5 for 2

|1

+1card or 6-10 for 4

|2

+ 2card
So how efficient is Devour? It depends on the hp of the Devouring creature.
Otyugh only affects 1-2 hp creatures
Elite Otyugh only affects 1-4hp creatures
Scarab requires swarming to be able to devour anything.
Consider 2 Lightnings vs 2 Scarabs
2 Lightnings (4

+ 2 cards) can kill 2 1-5hp creatures or 1 6-10hp creature
2 Scarabs (4

+ 2 cards) can kill 1hp creatures for 1

Or you could use Pharaoh
X Scarabs (9+2X

+ 1 card + X turns) can kill creatures with less than Xhp for 1

So for 17

+ 1 card + 4 turns you can kill creatures with 1-3 hp for 1

Owl's Eye (5

+ 1 card + 1 turn) can deal 3 damage for 2

So you can pay a cost of 22 to get "

:Kill 1-2 hp creature and gain +1|+1"
or you can pay a cost of 7 to get "

:Deal 3 damage to target creature"
7 << 22