Considered in broad context (meaning does it work across IOS devices and macOS devices), Photostream does not work well. Photostream usually works well between my iPhone 6 and iPad Pro 9.7, running the latest version of IOS. Photostream does not work consistently between the iPhone and iMac, however. Photos taken with the iPhone appear quickly in the iPad but not in the iMac. Sometimes the photos will appear in the iMac several days later, sometimes not at all. Usually deselecting, then re-selecting My Photostream in System Preference-iCloud-Photos-Options will cause Photostream to send photos to the iMac for a while, until it stops working again. At best, this is an inelegant solution that does not work that well.
I do not use iCloud Photo Library - I have too many photos for my upgraded 50GB iCloud storage tier.
I have called Apple technical support (my iMac is still under Apple Care) several times, but no solutions have been forthcoming.
My conclusion is that Photostream is one of those iCloud service that simply does not work reliably, at least between the iPhone and Macs.