Complete black-box. Impossible to work with. Impossible to get documentation. We literally had to debug via crash reports delivered to us every 24h. Of course it was ALWAYS their machine images somehow differing from their environments, so our tests passed locally and then failed in their black box.
Horrific performance problems (think services locking up at 200 concurrent connections...for mobile apps, when advertising-apps were still a thing. Released across virtually all pubs in a whole country. You can imagine how that went).
Of course they would continuously blame all their issues on the company I worked for at the time. And Diageo even fought tooth and nail to defend them. We had to waste an insane amount of resources just proving to them that their system sucked. Easily one of the worst examples of a company wasting huge amounts of money, thinking that outsourcing would save them money. It's so stupid.