One of the good/bad things about Flask is it's quite a small framework which lets the developer do whatever they want (yay!) so it's easy to make an unholy mess (boo!). I hope that I've stayed on the rails with PieFed.
It's a bit sad what has happened in the most recent version of SQLAlchemy - they made it all weird and difficult. PieFed uses an older version which I find very intuitive and simple.