The primary use case of this method is to facilitate partner account creation/updating within channels that have a customer billing relationship with Roku.
GetPartialUserData has just started working for us in our private channels. We received an email from Roku yesterday stating that our enrollment in Roku billing was complete ("Congratulations, your enrollment in Roku’s Billing Services Program is now complete." etc.).
That's about 1.5 - 2 months after we submitted our information so if you're having a similar issue, I'd suggest checking to see if you've received this confirmation email. Before this message came in it looked to me that we were already enrolled and everything was good to go. I didn't realize the enrollment wasn't finalized.