I have just spent a few hours day or so migrating subscriptions from aMember+phpBB to Stripe+Discourse Subscriptions. Some email addresses where off and I just spent an hour manually correcting them.
To double check, I tried to access the u/username/billing/subscriptions route for these users as an admin, but that does not work, unlike all the other u/username routes like activity, notifications, badges and preferences, so I had to impersonate these users one by one.
It would be very handy if an administrator would have the possibility to see the actual subscriptions for a user here.