I've tried sending a request to "/_api/ProjectServer/Projects('projectid')/Assignments", but all it does is return duplicates of the last assignment which is weird because the number of objects it returns is always equal to the number of assignments there are in the project.
Basically if I assign a resource to each of a hundred different tasks, the call returns 100 duplicates of the last task's assignment in the list.
I suspect it might be a bug, I'd appreciate it if someone could confirm or deny my assumption and/or let me know if there's any other way to retrieve the list of assignments in a project.