Given that dogfood and catfood work as far as mono-diets go
They mostly seem to, but if they cause a drop in energy or cognitive capability because of some nutrient balance problems, the animals won't become visibly ill and humans are unlikely to notice. A persistent brain fog from eating a poor diet would be quite bad for humans on the other hand.
Most of the selective breeding has been done while these animals were on simple diets, so perhaps some genetic adaptation has happened as well. Besides, aren't carnivore diets quite monotonous in nature anyway?
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.