Why would the inbound be more complex? Inbound they're basically taking credit cards, and while they probably have a few card processors they use and some complexity around taking different cards via different processors in different countries, it's not too complex.

Outbound, they're paying into bank accounts (more complex), in many different countries with different protocols for bank transfers, with significant reporting and auditing requirements, invoicing, etc. Also paying on different timescales for different things – whether bulk purchasing inventory, paying for on-demand bookings, etc. Payment terms will vary by many factors including who the payee is. Some payees like large chains may have their own accounting systems that need to be integrated with.

I'd have generally assumed that outbound would be much more complex!