I don't know, I've had ridiculous success with mono-gods back in my 7th Ed. demon days. It wasn't every day you can stack a leadership raping army so bad you make half the army run based on a terror check reaction, while the other gets Siren Songed and killed off piecemeal. Pure Slaaneshi demons was utter filth with Banner of Despair, as many Siren Songs as you can take, and Masque with Phantasmagoria. My mono Khorne army literally just pushed up with Juggernaught 0+ save Heralds, BT, Flesh Hounds and blocks of Bloodletters and killed everyone, it wasn't even fair. Nurgle just never died and attritioned every army to death, while mono Tz lists had like 36 PD on average. All of these mono-god armies won games, so that would mean they weren't limited and queued in as hammers, support and magic. They just won, and won hard because the book was powerful, all the units were good, and everything was well-priced.
8th Ed. Demons is better you say? Nurgle is insane, Beasts are severely undercosted, Skullcannons are the most laughably balanced unit in all of Warhammer, while Tzeentch and Khorne have been reduced to utter trash. Sure, it paved the way to randomness and fluffy fun, but also a huge degree of inconsistency and piss poor internal balance.
Regardless, I think GW is worse with Mat Ward. If they got rid of him, they made a big mistake. If they didn't, well, it's a huge loss to the design team because he was the one that really knew how to internally balance books. I agree that Phil Kelly is the man, as he basically has Mat Ward's sense of internal balance with respect to other armies. From what I've seen, Mat just wants his book to be good, with tons of player options, but without much consideration about the metagame.