There are several types of API testing that should be executed depending the objectives. API is critical for ensuring quality, resilience and performance.
The buy vs build decision includes cost, however it is not the only aspect. Reliability, performance, security, customization, and maintenance also play a role.
There are certain software engineering techniques that improve software quality and ensure long-term fitness of software systems.
Systems coupling has several dimensions. Hence, decoupling each aspect can take you in a different direction with conflicting trade-offs.