Mono god are playable and Nurgle is currently the strongest monogod list you can build.
monogod tzeentch may be hard as you ll have no combat troup really.
The main advantage of running monogod is that all you units can use the general leadership and the bsb rerolls which you can t otherwise. You lower the chance of having the reign of chaos table hurt your own units during the magic phase as 3/4 of the time your units will be safe when the opponent not so much.
Con is you ll miss on a couple of hard to pass units in the current book.
Beasts of Nurgle, Skullcannon of khorne are the main ones.
Competitive monogod lists usually play those units as a count as with model looking like they belong to the god they have chosen.
Those new mounted daemons on giant beasts look sick. I m going to build one for display