Daemons have a moronic book with a bajillion random tables you're supposed to roll on for days and days on end. If you like the idea of minimising the control you have over your own army, then this codex may be the one for you. It's not underpowered, mind you, just really fucking stupid.
Tau are strong, and are one of only three codices in the game with decent anti air available. They have solid troops and answers for just about every problem your opponent can throw at you. Perhaps most importantly, Tau don't have any goddamn tables apart from Warlord traits to roll on (IIRC).
"Knowledge is knowing tomato is a fruit. Wisdom is not putting it in fruit salad."
Also known as BaselessElm