I've discovered a very irritating hang-up bug. The following code demonstrates it:
v = ui.View()
im = ui.ImageView()
im.flex = 'LRTBWH'
Simply run this, then exit, then run again. Pythonista will hang up the second time. The only way to fix it is to multitask and force pythonista to close. This was happening in a larger program. I've fixed it in my program by loading images separately and then viewing the data (slower and lengthy code).
Smaller bug: If you start running from landscape mode, neither portrait or landscape UI will have the little clock bar at the top, causing the dimensions to be different than if you started from portrait.
Also, if there was a way to have an action run on orientation change, that would be great. Currently I have an "ori" ButtonItem at the top that recalculates the frames based on the parent frame dimensions, and it has to be manually pushed.