I think the biggest... thing with Piety is that it's meant to be use in conjunction with a Religion. Now the reason I didn't say "problem" or "strength" above is because it's kind of a double edged sword. The benefits of the universally recognized "better" trees, Tradition and Liberty, don't require you do do anything but select them. Piety helps you get a religion, which can mitigate the lost benefits of Tradition and Liberty. There are plenty of Production, Food, Gold, and even Science religious beliefs to chose from and Piety helps get you them faster.
The problem with going Piety and using Religion to fill in the gaps left by Tradition and Liberty is that you need a Religion. To get a food benefit from Tradition or production boost from Liberty, all you have to do is stay alive and get enough culture to adopt them. To get food bonuses from Piety, you need to build a shrine, wait till you get to adopt a pantheon, hope the one you want is still available, wait to found a Religion, then hope those beliefs are still available. Once you finally get a Religion, you need to spread it to your cities (which may not even get the bonus until they hit a certain Population), and then try to maintain it. Meanwhile, you're still going through Piety and missing out on free buildings, enhanced production, stronger military units, etc.
As many people have said, Piety is a great second tree. It allows you to use your religion to enhance your other bonuses as opposed to using religion for them. I honestly think two policy tweaks to Piety would make it a much stronger tree.
Ideally, I would get rid of Religious Tolerance and change it to "Your religion spreads to your cities 10% faster. +10% Culture in your cities following your religion"
It would also be kind of cool to add a % bonus to Faith either during Golden Ages or We Love the King Day, at least in the Holy City. As it stands now, you really aren't generating a ton of extra Faith through Piety (only 2 more per city than if you skipped Piety).
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