Interface testing is very annoying as the documentation is almost always lacking. However once the documents are complete it shouldn't be much too complicated, more like time consuming.

End-to-end testing spanning multiple systems and multiple interfaces is whole other ballgame. It's not just complex, but it's very intensive as you need to know where the issue has arisen. It's a lot of communication and hustle.

System tests and even interface tests can be carried out by developers. It's just that they're not too keen on doing it. And it should never be done by the people that built the damn thing. Out of obvious reasons.