The google play subscription API is probably the worst I ever had to integrate with.

For example

* Response leaves out historic data (i.e. a subscription doesn't show it was paused, had a trial, etc. once they're over)

* Fetching the subscription state frequently returns data inconsistent with previous results (e.g. the start date of the subscription keeps changing, the expiry date keeps jumping around), so piecing together a subscription's history by combining versions observed at different times is practically impossible

* Each subscription is a singly linked list, but doesn't give you enough information to actually follow the link. And it's from newest to oldest

* Authentication is a mess (it's half integrated into the google cloud, but not really)

* Settings changes take about ~24h to take effect