Piety is great for culture. Same as rationalism is great for science. So do you want more culture or more science?
I know that may be overly simplified but in strikes pretty close to my game experiences. if you look at piety, it has a lot of dud policies. The opener, mandate of heaven, free religion, reformation, and the finisher all solely boost culture (and a free GA). If you complete the tree you also get +10% gold and +2

/city. +10% gold is a joke. It's a way worse than building a market. There's no reason to take it besides wanting to finish the entire tree. Mandate of heaven is also rather useless but it's another must have if you want to finish the tree.
So basically piety gives you lots of +

and +2

/city.
Rationalism on the other hand, doesn't really have a dud policy. It provides the same amount of happiness per city (albeit somewhat later) and pushes your science forward significantly.
Basically if you want science (all victory conditions but culture) rationalism plays out better.
I know a lot of people make the argument that piety is great for happiness and I don't deny that it does provide a nice earlier boost (earlier than rationalism at least). However if you can learn to play without relying on that boost, you'll be far better off targetting rationalism for most situations.
Consider for victory conditions other than culture:
1) if you are a small empire you don't really need the extra happy that desperately so rationalism is better.
2) if you are a wide puppet / conquered empire, then honor will provide you more happy than piety and will also help you win your wars. So again honor followed by rationalism is better
3) if you are a wide, hard built empire, you are going to have massive policy costs. Dumping a bunch of policies into piety that do nothing but increase your culture is actually going to make it harder to take any future policies that provide any real benefit to you. You would've been better off taking a handful of policies in other trees rather than bother with piety. In this case order or honor or rationalism would be your best happy + extra branches.