This would not be such a serious problem if going piety was as good as Tradicion/Liberty, and if piety actually gave you fast religion. But Piety isnt fast way to religion.
Having UA/UB perk, or Liberty finisher, or Stonehage, or mix of ICS and luck with pantheon is the way to go. Since prophet from Piety comes so late, and tree itself dont grant culture it is inferior.
Yep. Although it's not so bad if you have a culture pantheon. The inherent faith boost from the tree kind of makes up for any lost speed in founding a religion. Obviously, culture/faith pantheons would be ideal, but those are really situational.
I also find the prophet finisher really awkward. I've tried Piety a bunch of times, and the best I can say for that last prophet is that he's a free Enhancer if you'd rather spend your early faith on missionaries, Pagodas, etc. and you don't care what beliefs are left when you get him--this can actually be quite strong, because if you want to spread your religion you'll have a huge head start, or you can boom your faith/culture income earlier with religious buildings (which has the added benefit of popping your borders, since there's less culture from normal buildings now). If you do care about your last beliefs, it's always going to be faster to hit 300 faith, and then the final prophet is basically just a holy site or a little extra spread, which isn't a very good use of the finisher.
Also, holy sites are not good tiles to work even with all the bonuses from the tree unless it's very late in the game or you're going for a tourism win. Early on, food and hammers are way more important.
I do kind of like Liberty (Collective Rule) / Piety if you have a good pantheon and aren't too particular about what Reformation belief you get. But then, there aren't very many good Reformations. Still, Piety in the Ancient era is a good change, and I'm sure the fall patch can whip it into shape a bit. An earlier prophet might be all the tree really needs.