How do Aperture stacks work in the Photos app?
Here's what I know about Photos Version 1.0 (209.52.0):


Albums you make in Photos are only sortable by date ascending or manually (drag the photos in the order you wish)
Shared Albums are not sortable - last in on the bottom
The All Photos album isn't sortable - last in on the bottom (and imports are multi-threaded, so don't expect photos to be arranged in any order like they were on the filesystem when you let the app loose on an import of more than one photo.)
Burst photos seem to stay grouped (most of the time) and Aperture stacks do not ever stay grouped.


Basically, the rule is you don't even have a sort option enabled and when you do, it's one direction by time so you get one default sort to sometimes enforce in Photos 1.0